fix(errata): workaround for Cortex-A78 erratum 2395406

Cortex-A78 erratum 2395406 is a cat B erratum that applies to revisions
r0p0 - r1p2 and is still open. The workaround is to set bit[40] of
CPUACTLR2 which will disable folding of demand requests into older
prefetches with L2 miss requests outstanding.

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

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: If06f988f05f925c2a4bed3e6a9414b6acdfec894
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index 2a47abe..1ad662f 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -300,6 +300,10 @@
    CPU. This needs to be enabled for revisions r0p0, r1p0, r1p1, and r1p2, and
    it is still open.
 
+-  ``ERRATA_A78_2395406``: This applies errata 2395406 workaround to Cortex-A78
+   CPU. This needs to be enabled for revisions r0p0, r1p0, r1p1, and r1p2, and
+   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