fix(cpus): workaround for Cortex-X2 erratum 2136059

Cortex-X2 erratum 2136059 is a Cat B erratum that applies to
revisions r0p0, r1p0 and r2p0 and is fixed in r2p1.

The workaround is to set CPUACTLR5_EL1[44].

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

Change-Id: I156467537c3f235b50fc8aa19a969f2798bd891b
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 515cde3..430dc04 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -774,6 +774,10 @@
    CPU. This needs to be enabled only for revisions r0p0, r1p0 and r2p0 of the
    CPU, it is fixed in r2p1.
 
+-  ``ERRATA_X2_2136059``: This applies errata 2136059 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_2216384``: This applies errata 2216384 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.