fix(cpus): workaround for Cortex X3 erratum 2641945

Cortex X3 erratum 2641945 is a Cat B erratum that applies to all
revisions <= r1p0 and is fixed in r1p1.

The workaround is to disable the affected L1 data cache prefetcher
by setting CPUACTLR6_EL1[41] to 1. Doing so will incur a performance
penalty of ~1%. Contact Arm for an alternate workaround that impacts
power.

SDEN documentation:
https://developer.arm.com/documentation/2055130/latest

Change-Id: Ia6d6ac8a66936c63b8aa8d7698b937f42ba8f044
Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index 3485dc9..652c142 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -791,6 +791,10 @@
   CPU. This needs to be enabled only for revisions r0p0, r1p0 and r1p1 of the
   CPU, it is still open.
 
+- ``ERRATA_X3_2641945``: This applies errata 2641945 workaround to Cortex-X3
+  CPU. This needs to be enabled only for revisions r0p0 and r1p0 of the CPU.
+  It is fixed in r1p1.
+
 - ``ERRATA_X3_2742421``: This applies errata 2742421 workaround to
   Cortex-X3 CPU. This needs to be enabled only for revisions r0p0, r1p0 and
   r1p1. It is fixed in r1p2.