fix(cpus): workaround for Cortex-A710 erratum 2768515

Cortex-A710 erratum 2768515 is a Cat B erratum that applies to
all revisions <=r2p1 and is still open. The workaround is to
insert a dsb before the isb in the power down sequence.

SDEN documentation:
https://developer.arm.com/documentation/SDEN1775101/latest

Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: If17fe04d3fda0dba6b8aabdd837a1c53e1830ed5
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index d48f284..368da0a 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -518,6 +518,10 @@
    Cortex-A710 CPU. This needs to be enabled for revisions r0p0, r1p0 and r2p0
    of the CPU and is fixed in r2p1.
 
+-  ``ERRATA_A710_2768515``: This applies errata 2768515 workaround to
+   Cortex-A710 CPU. This needs to be enabled for revisions r0p0, r1p0, r2p0 and
+   r2p1 of the CPU and is still open.
+
 For Neoverse N2, the following errata build flags are defined :
 
 -  ``ERRATA_N2_2002655``: This applies errata 2002655 workaround to Neoverse-N2