fix(errata): workaround for Cortex A78 erratum 2242635

Cortex A78 erratum 2242635 is a Cat B erratum present in the A78 Core.
It applies to revisions r1p0, r1p1, r1p2, and is still open. The issue
is also present in r0p0 but there is no workaround for this revision.

SDEN can be found here:
https://developer.arm.com/documentation/SDEN1401784

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: Ieca024254cabbc683ff13a70f3aeb8f2f3c5ce07
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index b5ec363..1d4f67a 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -288,6 +288,10 @@
    CPU. This needs to be enabled for revisions r0p0, r1p0, r1p1, and r1p2. It
    is still open.
 
+-  ``ERRATA_A78_2242635``: This applies errata 2242635 workaround to Cortex-A78
+   CPU. This needs to be enabled for revisions r1p0, r1p1, and r1p2. The issue
+   is present in r0p0 but there is no workaround. It is still open.
+
 For Cortex-A78 AE, the following errata build flags are defined :
 
 - ``ERRATA_A78_AE_1941500`` : This applies errata 1941500 workaround to Cortex-A78