fix(cpus): workaround for Cortex-X2 erratum 2742423

Cortex-X2 erratum 2742423 is a Cat B erratum that applies to all
revisions <= r2p1 and is still open.
The workaround is to set CPUACTLR5_EL1[56:55] to 2'b01.

SDEN documentation:
https://developer.arm.com/documentation/SDEN-1775100/latest

Change-Id: I03897dc2a7f908937612c2b66ce7a043c1b7575d
Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
diff --git a/lib/cpus/cpu-ops.mk b/lib/cpus/cpu-ops.mk
index 8b167a8..434ee08 100644
--- a/lib/cpus/cpu-ops.mk
+++ b/lib/cpus/cpu-ops.mk
@@ -743,6 +743,10 @@
 # and is still open.
 CPU_FLAG_LIST += ERRATA_X2_2701952
 
+# Flag to apply erratum 2742423 workaround during reset. This erratum applies
+# to revisions r0p0, r1p0, r2p0 and r2p1 of the Cortex-X2 cpu and is still open.
+CPU_FLAG_LIST += ERRATA_X2_2742423
+
 # 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.