fix(cpus): workaround for Cortex-A720 erratum 2940794

Cortex-A720 erratum 2940794 is a Cat B erratum that is present
in revision r0p0, r0p1 and is fixed in r0p2.

The workaround is to set bit[37] of the CPUACTLR2_EL1 to 1.

SDEN documentation:
https://developer.arm.com/documentation/SDEN2439421/latest

Change-Id: I1488802e0ec7c16349c9633bb45de4d0e1faa9ad
Signed-off-by: Bipin Ravi <biprav01@u203721.austin.arm.com>
(cherry picked from commit 7385213e602465d27530015a9b28ebc36a77b1c1)
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index 97e164d..2f590b2 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -916,6 +916,11 @@
    Cortex-A715 CPU. This needs to be enabled only for revision r1p0.
    It is fixed in r1p1.
 
+For Cortex-A720, the following errata build flags are defined :
+
+-  ``ERRATA_A720_2940794``: This applies errata 2940794 workaround to
+   Cortex-A720 CPU. This needs to be enabled for revisions r0p0 and r0p1.
+   It is fixed in r0p2.
 
 DSU Errata Workarounds
 ----------------------