fix(cpus): workaround for Cortex-X2 erratum 2778471

Cortex-X2 erratum 2778471 is a Cat B erratum that applies
to revisions r0p1, r1p0, r2p0 and r2p1 and is still open.
The workaround is to set CPUACTLR3_EL1[47] to 1.

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

Change-Id: Ia95f0e276482283bf50e06c58c2bc5faab3f62c6
Signed-off-by: Sona Mathew <sonarebecca.mathew@arm.com>
diff --git a/lib/cpus/cpu-ops.mk b/lib/cpus/cpu-ops.mk
index 72a6a20..a0f43d5 100644
--- a/lib/cpus/cpu-ops.mk
+++ b/lib/cpus/cpu-ops.mk
@@ -765,6 +765,10 @@
 # still open.
 CPU_FLAG_LIST += ERRATA_X2_2768515
 
+# Flag to apply erratum 2778471 workaround during reset. This erratum applies
+# to revisions r0p0, r1p0, r2p0, r2p1 of the Cortex-X2 cpu and it is still open.
+CPU_FLAG_LIST += ERRATA_X2_2778471
+
 # Flag to apply erratum 2070301 workaround on reset. This erratum applies
 # to revisions r0p0, r1p0, r1p1 and r1p2 of the Cortex-X3 cpu and is
 # still open.