fix(cpus): workaround for Cortex-X4 erratum 2923985

Cortex-X4 erratum 2923935 is a Cat B erratum that applies
to all revisions <= r0p1 and is fixed in r0p2.

The workaround is to set CPUACTLR4_EL1[11:10] to 0b11.

SDEN documentation:
https://developer.arm.com/documentation/SDEN-2432808/latest

Signed-off-by: Arvind Ram Prakash <arvind.ramprakash@arm.com>
Change-Id: I9207802ad479919a7f77c1271019fa2479e076ee
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index fda43dc..f1cb73b 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -843,6 +843,9 @@
 - ``ERRATA_X4_2897503``: This applies errata 2897503 workaround to Cortex-X4
   CPU. This needs to be enabled for revisions r0p0 and r0p1. It is fixed in r0p2.
 
+- ``ERRATA_X4_2923985``: This applies errata 2923985 workaround to Cortex-X4
+  CPU. This needs to be enabled for revisions r0p0 and r0p1. It is fixed in r0p2.
+
 - ``ERRATA_X4_3076789``: This applies errata 3076789 workaround to Cortex-X4
   CPU. This needs to be enabled for revisions r0p0 and r0p1. It is fixed in r0p2.