fix(cpus): workaround for Cortex-X2 erratum 2291219

Cortex-X2 erratum 2291219 is a Cat B erratum that applies to
revisions r0p0, r1p0 and r2p0 and is fixed in r2p1.

The workaround is to set CPUACTLR2_EL1[36] before the power
down sequence that sets PWRDN_EN and executes WFI. This bit
should be be cleared after exiting WFI.

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

Change-Id: I43af57961feba3a1c001d09ad804740b996f1db7
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 77549ac..d47d082 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -794,6 +794,10 @@
    CPU. This needs to be enabled for revisions r0p0, r1p0, r2p0 and r2p1 of the
    CPU and is still open.
 
+-  ``ERRATA_X2_2291219``: This applies errata 2291219 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_2371105``: This applies errata 2371105 workaround to Cortex-X2
    CPU. This needs to be enabled for revisions r0p0, r1p0 and r2p0 of the CPU
    and is fixed in r2p1.