Workaround for Cortex A76 erratum 1946160

Cortex A76 erratum 1946160 is a Cat B erratum, present in some revisions
of the A76 processor core.  The workaround is to insert a DMB ST before
acquire atomic instructions without release semantics.  This issue is
present in revisions r0p0 - r4p1  but this workaround only applies to
revisions r3p0 - r4p1, there is no workaround for older versions.

SDEN can be found here:
https://documentation-service.arm.com/static/5fbb77d7d77dd807b9a80cc1

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: Ief33779ee76a89ce2649812ae5214b86a139e327
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index e901e0c..d859cc5 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -249,6 +249,9 @@
 -  ``ERRATA_A76_1868343``: This applies errata 1868343 workaround to Cortex-A76
    CPU. This needs to be enabled only for revision <= r4p0 of the CPU.
 
+-  ``ERRATA_A76_1946160``: This applies errata 1946160 workaround to Cortex-A76
+   CPU. This needs to be enabled only for revisions r3p0 - r4p1 of the CPU.
+
 For Cortex-A77, the following errata build flags are defined :
 
 -  ``ERRATA_A77_1508412``: This applies errata 1508412 workaround to Cortex-A77