fix(errata): workaround for Cortex-X2 erratum 2147715

Cortex-X2 erratum 2147715 is a Cat B erratum that applies to revision
r2p0 and is fixed in r2p1. The workaround is to set CPUACTLR_EL1[22]=1,
which will cause the CFP instruction to invalidate all branch predictor
resources regardless of context.

SDEN can be found here:
https://developer.arm.com/documentation/SDEN1775100/latest

Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: I2d81867486d9130f2c36cd4554ca9a8f37254b57
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index af0e769..3029458 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -490,6 +490,10 @@
    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_2147715``: This applies errata 2147715 workaround to
+   Cortex-X2 CPU. This needs to be enabled only for revision r2p0 of the CPU,
+   it is fixed in r2p1.
+
 For Cortex-A510, the following errata build flags are defined :
 
 -  ``ERRATA_A510_1922240``: This applies errata 1922240 workaround to