fix(cpus): workaround for Cortex-X4 erratum 2897503

Cortex-X4 erratum 2897503 is a Cat B erratum that applies
to all revisions <= r0p1 and is fixed in r0p2.

The workaround is to set CPUACTLR4_EL1[8] to 1.

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

Signed-off-by: Arvind Ram Prakash <arvind.ramprakash@arm.com>
Change-Id: I3178a890b6f1307b310e817af75f8fdfb8668cc9
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index e6ca542..5230cdc 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -840,6 +840,9 @@
 - ``ERRATA_X4_2816013``: This applies errata 2816013 workaround to Cortex-X4
   CPU. This needs to be enabled for revisions r0p0 and r0p1. It is fixed in r0p2.
 
+- ``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.
+
 For Cortex-A510, the following errata build flags are defined :
 
 -  ``ERRATA_A510_1922240``: This applies errata 1922240 workaround to