fix(cpus): workaround for Cortex-X2 erratum 2267065

Cortex-X2 erratum 2267065 is a Cat B erratum that applies to
revisions r0p0, r1p0 and r2p0 and is fixed in r2p1.

The workaround is to set CPUACTLR_EL1[22].

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

Change-Id: I540e113f209ef11ec7103d4ef4e48ffb52416b4e
Signed-off-by: John Powell <john.powell@arm.com>
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index 430dc04..77549ac 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -786,6 +786,10 @@
    CPU. This needs to be enabled only for revision r2p0 of the CPU, it is fixed
    in r2p1.
 
+-  ``ERRATA_X2_2267065``: This applies errata 2267065 workaround to Cortex-X2
+   CPU. This needs to be enabled only for revisions r0p0, r1p0 and r2p0 of the
+   CPU, it is fixed in r2p1.
+
 -  ``ERRATA_X2_2282622``: This applies errata 2282622 workaround to Cortex-X2
    CPU. This needs to be enabled for revisions r0p0, r1p0, r2p0 and r2p1 of the
    CPU and is still open.