chore(docs): update march utility details

commit@7794d6c8f8c44acc14fbdc5ada5965310056be1e added a march utility
but the details were not updated in docs.

Update docs to provide a glimpse of march utility added.

Change-Id: I696cb9a701a30d7bf36a1ecd38a80d07df1fd551
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>

docs/design/firmware-design.rst

@@ -2636,19 +2636,17 @@ These build options serve dual purpose
   architectural features up to ``ARM_ARCH_MAJOR.ARM_ARCH_MINOR`` are included
   and unconditionally enabled by TF-A build system.
 
-- Passed to compiler via "-march" option to generate binary target : Tell the
-  compiler to emit instructions upto ``ARM_ARCH_MAJOR.ARM_ARCH_MINOR``
+- ``ARM_ARCH_MAJOR`` and ``ARM_ARCH_MINOR`` are passed to a march.mk build utility
+  this will try to come up with an appropriate -march value to be passed to compiler
+  by probing the compiler and checking what's supported by the compiler and what's best
+  that can be used. But if platform provides a ``MARCH_DIRECTIVE`` then it will used
+  directly and compiler probing will be skipped.
 
 The build system requires that the platform provides a valid numeric value based on
 CPU architecture extension, otherwise it defaults to base Armv8.0-A architecture.
 Subsequent Arm Architecture versions also support extensions which were introduced
 in previous versions.
 
-**TO-DO** : Its planned to decouple the two functionalities and introduce a new macro
-for compiler usage. The requirement for this decoupling arises becasue TF-A code
-always provides support for the latest and greatest architecture features but this
-is not the case for the target compiler.
-
 .. seealso:: :ref:`Build Options`
 
 For details on the Architecture Extension and available features, please refer
@@ -2738,7 +2736,7 @@ I.e:
 
 .. code:: make
 
-   MARCH_DIRECTIVE := -mach=armv7-a
+   MARCH_DIRECTIVE := -march=armv7-a
 
 Code Structure
 --------------

docs/getting_started/build-options.rst

@@ -714,6 +714,13 @@ Common build options
 
    This option defaults to 0.
 
+-  ``MARCH_DIRECTIVE``: used to pass a -march option from the platform build
+   options to the compiler. An example usage:
+
+   .. code:: make
+
+      MARCH_DIRECTIVE := -march=armv8.5-a
+
 -  ``NON_TRUSTED_WORLD_KEY``: This option is used when ``GENERATE_COT=1``. It
    specifies the file that contains the Non-Trusted World private key in PEM
    format. If ``SAVE_KEYS=1``, this file name will be used to save the key.