docs: marvell: Update info about WTMI_IMG option

Default WTMI_IMG value was documented incorrectly. Also WTMI_IMG name may
be misleading as this option does not specify full WTMI image, just a main
loop (e.g. fuse.bin or custom RTOS image) without hardware initialization
code (DDR, CPU and clocks).

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I3de4a27ce2165b962fa628c992fd8f80151efd7c

docs/plat/marvell/armada/build.rst

@@ -185,12 +185,17 @@ There are several build options:
 
 - WTMI_IMG
 
-        For Armada37x0 only, the path of the WTMI image can point to an image which
+        For Armada37x0 only, the path of the binary can point to an image which
         does nothing, an image which supports EFUSE or a customized CM3 firmware
-        binary. The default image is wtmi.bin that built from sources in WTP
+        binary. The default image is ``fuse.bin`` that built from sources in WTP
         folder, which is the next option. If the default image is OK, then this
         option should be skipped.
 
+        Please note that this is not a full WTMI image, just a main loop without
+        hardware initialization code. Final WTMI image is built from this WTMI_IMG
+        binary and sys-init code from the WTP directory which sets DDR and CPU
+        clocks according to DDR_TOPOLOGY and CLOCKSPRESET options.
+
 - WTP
 
         For Armada37x0 only, use this parameter to point to wtptools source code