fix(cpus): workaround for Cortex-X2 erratum 2768515

Cortex-X2 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/SDEN1775100/latest

Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: Ib02688f7b6dc7f6ec305e68e8895174f6fd577a0
diff --git a/lib/cpus/cpu-ops.mk b/lib/cpus/cpu-ops.mk
index 3db86dd..527a82f 100644
--- a/lib/cpus/cpu-ops.mk
+++ b/lib/cpus/cpu-ops.mk
@@ -660,6 +660,11 @@
 # to revision r0p0, r1p0 and r2p0 of the Cortex-X2 cpu and is fixed in r2p1.
 ERRATA_X2_2371105	?=0
 
+# Flag to apply erratum 2768515 workaround during power down. This erratum
+# applies to revision r0p0, r1p0, r2p0 and r2p1 of the Cortex-X2 cpu and is
+# still open.
+ERRATA_X2_2768515	?=0
+
 # Flag to apply erratum 2313909 workaround on powerdown. This erratum applies
 # to revisions r0p0 and r1p0 of the Cortex-X3 cpu, it is fixed in r1p1.
 ERRATA_X3_2313909	?=0
@@ -1297,6 +1302,10 @@
 $(eval $(call assert_boolean,ERRATA_X2_2371105))
 $(eval $(call add_define,ERRATA_X2_2371105))
 
+# Process ERRATA_X2_2768515 flag
+$(eval $(call assert_boolean,ERRATA_X2_2768515))
+$(eval $(call add_define,ERRATA_X2_2768515))
+
 # Process ERRATA_X3_2313909 flag
 $(eval $(call assert_boolean,ERRATA_X3_2313909))
 $(eval $(call add_define,ERRATA_X3_2313909))