docs: add build options for GPT support enablement

Documented the build options used in Arm GPT parser enablement.

Change-Id: I9d7ef2f44b8f9d2731dd17c2639e5ed0eb6d0b3a
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>

docs/plat/arm/arm-build-options.rst

@@ -104,6 +104,14 @@ Arm Platform Build Options
    device tree. This flag is defined only when ``ARM_SPMC_MANIFEST_DTS`` manifest
    file name contains pattern optee_sp.
 
+-  ``ARM_GPT_SUPPORT``: Enable GPT parser to get the entry address and length of
+   the various partitions present in the GPT image. This support is available
+   only for the BL2 component, and it is disabled by default.
+   The following diagram shows the view of the FIP partition inside the GPT
+   image:
+
+   |FIP in a GPT image|
+
 For a better understanding of these options, the Arm development platform memory
 map is explained in the :ref:`Firmware Design`.
 
@@ -140,4 +148,6 @@ Arm CSS Platform-Specific Build Options
 
 --------------
 
+.. |FIP in a GPT image| image:: ../../resources/diagrams/FIP_in_a_GPT_image.png
+
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