docs: re-parent BL2 platform hooks for measured boot

bl2_plat_mboot_init/finish() functions documentation was incorrectly
hooked up to BL2U-specific section.

Change-Id: I758cb8142e992b0c85ee36d5671fc9ecd5bde29b
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

docs/getting_started/porting-guide.rst

@@ -1665,6 +1665,42 @@ element in the boot sequence. If there are no more boot sources then it
 must return 0, otherwise it must return 1. The default implementation
 of this always returns 0.
 
+Function : bl2_plat_mboot_init() [optional]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+    Argument : void
+    Return   : void
+
+When the MEASURED_BOOT flag is enabled:
+
+-  This function is used to initialize the backend driver(s) of measured boot.
+-  On the Arm FVP port, this function is used to initialize the Event Log
+   backend driver with the Event Log buffer information (base address and
+   size) received from BL1. It results in panic on error.
+
+When the MEASURED_BOOT flag is disabled, this function doesn't do anything.
+
+Function : bl2_plat_mboot_finish() [optional]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+    Argument : void
+    Return   : void
+
+When the MEASURED_BOOT flag is enabled:
+
+-  This function is used to finalize the measured boot backend driver(s),
+   and also, set the information for the next bootloader component to extend
+   the measurement if needed.
+-  On the Arm FVP port, this function is used to pass the Event Log buffer
+   information (base address and size) to non-secure(BL33) and trusted OS(BL32)
+   via nt_fw and tos_fw config respectively. It results in panic on error.
+
+When the MEASURED_BOOT flag is disabled, this function doesn't do anything.
+
 Boot Loader Stage 2 (BL2) at EL3
 --------------------------------
 
@@ -1822,42 +1858,6 @@ Application Processor (AP) for BL2U execution to continue.
 This function returns 0 on success, a negative error code otherwise.
 This function is included if SCP_BL2U_BASE is defined.
 
-Function : bl2_plat_mboot_init() [optional]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-::
-
-    Argument : void
-    Return   : void
-
-When the MEASURED_BOOT flag is enabled:
-
--  This function is used to initialize the backend driver(s) of measured boot.
--  On the Arm FVP port, this function is used to initialize the Event Log
-   backend driver with the Event Log buffer information (base address and
-   size) received from BL1. It results in panic on error.
-
-When the MEASURED_BOOT flag is disabled, this function doesn't do anything.
-
-Function : bl2_plat_mboot_finish() [optional]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-::
-
-    Argument : void
-    Return   : void
-
-When the MEASURED_BOOT flag is enabled:
-
--  This function is used to finalize the measured boot backend driver(s),
-   and also, set the information for the next bootloader component to extend
-   the measurement if needed.
--  On the Arm FVP port, this function is used to pass the Event Log buffer
-   information (base address and size) to non-secure(BL33) and trusted OS(BL32)
-   via nt_fw and tos_fw config respectively. It results in panic on error.
-
-When the MEASURED_BOOT flag is disabled, this function doesn't do anything.
-
 Boot Loader Stage 3-1 (BL31)
 ----------------------------