fix(cpus): workaround for Cortex-X2 erratum 3701772

Cortex-X2 erratum 3701772 that applies to r0p0, r1p0, r2p0, r2p1
is still Open.

The workaround is for EL3 software that performs context save/restore
on a change of Security state to use a value of SCR_EL3.NS when
accessing ICH_VMCR_EL2 that reflects the Security state that owns the
data being saved or restored.

SDEN documentation:
https://developer.arm.com/documentation/SDEN-1775100/latest/

Change-Id: I2ffc5e7d7467f1bcff8b895fea52a1daa7d14495
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index 57875da..00cfe19 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -782,6 +782,10 @@
    CPU. This needs to be enabled for revisions r0p0, r1p0, r2p0 and r2p1 of the
    CPU and it is still open.
 
+-  ``ERRATA_X2_3701772``: This applies errata 3701772 workaround to Cortex-X2
+   CPU. This needs to be enabled for revisions r0p0, r1p0, r2p0 and r2p1 of the
+   CPU and it is still open.
+
 For Cortex-X3, the following errata build flags are defined :
 
 - ``ERRATA_X3_2070301``: This applies errata 2070301 workaround to the Cortex-X3