fix(errata): workaround for Cortex-X2 errata 2017096

Cortex-X2 erratum 2017096 is a Cat B erratum that applies to
revisions r0p0, r1p0 & r2p0. The workaround is to set CPUECLTR_EL1[8]
to 1 which disables store issue prefetching.

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

Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: I3b740aedc95c2394f6b8d1186014d2b2f640ae05
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index 7075ca6..ef21050 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -454,6 +454,10 @@
 -  ``ERRATA_X2_2083908``: This applies errata 2083908 workaround to Cortex-X2
    CPU. This needs to be enabled for revision r2p0 of the CPU, it is still open.
 
+-  ``ERRATA_X2_2017096``: This applies errata 2017096 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.
+
 DSU Errata Workarounds
 ----------------------