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/lib/cpus/cpu-ops.mk b/lib/cpus/cpu-ops.mk
index 8ef794b..3db86dd 100644
--- a/lib/cpus/cpu-ops.mk
+++ b/lib/cpus/cpu-ops.mk
@@ -572,6 +572,11 @@
 # to revision r0p0, r1p0 and r2p0 of the Cortex-A710 cpu and is fixed in r2p1.
 ERRATA_A710_2371105	?=0
 
+# Flag to apply erratum 2768515 workaround during power down. This erratum
+# applies to revision r0p0, r1p0, r2p0 and r2p1 of the Cortex-A710 cpu and is
+# still open.
+ERRATA_A710_2768515	?=0
+
 # Flag to apply erratum 2067956 workaround during reset. This erratum applies
 # to revision r0p0 of the Neoverse N2 cpu and is still open.
 ERRATA_N2_2067956	?=0
@@ -1208,6 +1213,10 @@
 $(eval $(call assert_boolean,ERRATA_A710_2371105))
 $(eval $(call add_define,ERRATA_A710_2371105))
 
+# Process ERRATA_A710_2768515 flag
+$(eval $(call assert_boolean,ERRATA_A710_2768515))
+$(eval $(call add_define,ERRATA_A710_2768515))
+
 # Process ERRATA_N2_2067956 flag
 $(eval $(call assert_boolean,ERRATA_N2_2067956))
 $(eval $(call add_define,ERRATA_N2_2067956))