doc: Build option to protect GICR frame

Added a build option 'FVP_GICR_REGION_PROTECTION' to make
redistributor frame of fused/unused cores as read only.

Change-Id: Ie85f86e2465b93321a92a888ce8712a3144e4ccb
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>

docs/plat/arm/fvp/index.rst

@@ -142,6 +142,11 @@ Arm FVP Platform Specific Build Options
    HW_CONFIG blob instead of the DTS file. This option is useful to override
    the default HW_CONFIG selected by the build system.
 
+-  ``FVP_GICR_REGION_PROTECTION``: Mark the redistributor pages of
+   inactive/fused CPU cores as read-only. The default value of this option
+   is ``0``, which means the redistributor pages of all CPU cores are marked
+   as read and write.
+
 Booting Firmware Update images
 ------------------------------