fix(cpus): workaround for Cortex-A510 erratum 2684597

Cortex-A510 erratum 2684597 is a Cat B erratum that applies to revisions
r0p0, r0p1, r0p2, r0p3, r1p0, r1p1 and r1p2. It is fixed in r1p3. The
workaround is to execute a TSB CSYNC and DSB before executing WFI for
power down.

SDEN can be found here:
https://developer.arm.com/documentation/SDEN1873361/latest
https://developer.arm.com/documentation/SDEN1873351/latest

Change-Id: Ic0b24b600bc013eb59c797401fbdc9bda8058d6d
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
diff --git a/lib/psci/aarch64/psci_helpers.S b/lib/psci/aarch64/psci_helpers.S
index 0bb1988..61f31e5 100644
--- a/lib/psci/aarch64/psci_helpers.S
+++ b/lib/psci/aarch64/psci_helpers.S
@@ -124,6 +124,9 @@
  * -----------------------------------------------------------------------
  */
 func psci_power_down_wfi
+#if ERRATA_A510_2684597
+	bl apply_cpu_pwr_dwn_errata
+#endif
 	dsb	sy		// ensure write buffer empty
 1:
 	wfi