fix(cpus): workaround for Cortex-X3 erratum 2742421

Cortex-X3 erratum 2742421 is a Cat B erratum that applies to
all revisions <= r1p1 and is fixed in r1p2. The workaround is to
set CPUACTLR5_EL1[56:55] to 2'b01.

SDEN documentation:
https://developer.arm.com/documentation/2055130/latest

Signed-off-by: Sona Mathew <sonarebecca.mathew@arm.com>
Change-Id: Idadd323e419739fe909b9b68ea2dbe857846666b
diff --git a/lib/cpus/cpu-ops.mk b/lib/cpus/cpu-ops.mk
index 8df0a29..cbb0f20 100644
--- a/lib/cpus/cpu-ops.mk
+++ b/lib/cpus/cpu-ops.mk
@@ -738,6 +738,10 @@
 # to revisions r0p0, r1p0, r1p1 of the Cortex-X3 cpu, it is still open.
 CPU_FLAG_LIST += ERRATA_X3_2615812
 
+# Flag to apply erratum 2742421 workaround on reset. This erratum applies
+# to revisions r0p0, r1p0 and r1p1 of the Cortex-X3 cpu, it is fixed in r1p2.
+CPU_FLAG_LIST += ERRATA_X3_2742421
+
 # Flag to apply erratum 1922240 workaround during reset. This erratum applies
 # to revision r0p0 of the Cortex-A510 cpu and is fixed in r0p1.
 CPU_FLAG_LIST += ERRATA_A510_1922240