Invalidate dcache build option for bl2 entry at EL3

Some of the platform (ie. Agilex) make use of CCU IPs which will only be
initialized during bl2_el3_early_platform_setup. Any operation to the
cache beforehand will crash the platform. Hence, this will provide an
option to skip the data cache invalidation upon bl2 entry at EL3

Signed-off-by: Hadi Asyrafi <muhammad.hadi.asyrafi.abdul.halim@intel.com>
Change-Id: I2c924ed0589a72d0034714c31be8fe57237d1f06
diff --git a/docs/getting_started/user-guide.rst b/docs/getting_started/user-guide.rst
index 48cbdb9..3828d0b 100644
--- a/docs/getting_started/user-guide.rst
+++ b/docs/getting_started/user-guide.rst
@@ -287,6 +287,12 @@
    enable this use-case. For now, this option is only supported when BL2_AT_EL3
    is set to '1'.
 
+-  ``BL2_INV_DCACHE``: This is an optional build option which control dcache
+   invalidation upon BL2 entry. Some platform cannot handle cache operations
+   during entry as the coherency unit is not yet initialized. This may cause
+   crashing. Leaving this option to '1' (default) will allow the operation.
+   This option is only relevant when BL2_AT_EL3 is set to '1'.
+
 -  ``BL31``: This is an optional build option which specifies the path to
    BL31 image for the ``fip`` target. In this case, the BL31 in TF-A will not
    be built.