fix(cpus): workaround for Cortex-X2 erratum 2742423

Cortex-X2 erratum 2742423 is a Cat B erratum that applies to all
revisions <= r2p1 and is still open.
The workaround is to set CPUACTLR5_EL1[56:55] to 2'b01.

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

Change-Id: I03897dc2a7f908937612c2b66ce7a043c1b7575d
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 11e7992..d03daf8 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -733,6 +733,10 @@
    This needs to be enabled for revisions r0p0, r1p0, r2p0 and r2p1 and is
    still open.
 
+-  ``ERRATA_X2_2742423``: This applies errata 2742423 workaround to Cortex-X2
+   CPU. This needs to be enabled for revisions r0p0, r1p0, r2p0 and r2p1 of the
+   CPU and is still open.
+
 -  ``ERRATA_X2_2768515``: This applies errata 2768515 workaround to Cortex-X2
    CPU. This needs to be enabled for revisions r0p0, r1p0, r2p0 and r2p1 of the
    CPU and is still open.