Workaround for Cortex A77 erratum 1508412

Cortex A77 erratum 1508412 is a Cat B Errata present in r0p0 and r1p0.
The workaround is a write sequence to several implementation defined
registers based on A77 revision.

This errata is explained in this SDEN:
https://static.docs.arm.com/101992/0010/Arm_Cortex_A77_MP074_Software_Developer_Errata_Notice_v10.pdf

Signed-off-by: Lauren Wehrmeister <lauren.wehrmeister@arm.com>
Change-Id: I217993cffb3ac57c313db8490e7b8a7bb393379b
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index 3c0e30f..e9ff17e 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -251,6 +251,9 @@
 
 For Cortex-A77, the following errata build flags are defined :
 
+-  ``ERRATA_A77_1508412``: This applies errata 1508412 workaround to Cortex-A77
+   CPU. This needs to be enabled only for revision <= r1p0 of the CPU.
+
 -  ``ERRATA_A77_1800714``: This applies errata 1800714 workaround to Cortex-A77
    CPU. This needs to be enabled only for revision <= r1p1 of the CPU.