fix(cpus): workaround for Cortex-A520 erratum 2630792

Cortex-A520 erratum is a Cat B erratum that applies
to revisions r0p0 and r0p1 and is still open.
The workaround is to set CPUACTLR_EL1[38] to 1.

SDEN Documentation:
https://developer.arm.com/documentation/SDEN-2444153/latest

Change-Id: Idb6f32f680ee1378a57c2d2f809ea847fffe5910
Signed-off-by: Sona Mathew <sonarebecca.mathew@arm.com>
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index 11ab798..b29c752 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -840,6 +840,12 @@
    Cortex-A510 CPU. This needs to be applied to revision r0p0, r0p1, r0p2,
    r0p3, r1p0, r1p1 and r1p2. It is fixed in r1p3.
 
+For Cortex-A520, the following errata build flags are defined :
+
+-  ``ERRATA_A520_2630792``: This applies errata 2630792 workaround to
+   Cortex-A520 CPU. This needs to applied for revisions r0p0, r0p1 of the
+   CPU and is still open.
+
 For Cortex-A715, the following errata build flags are defined :
 
 -  ``ERRATA_A715_2701951``: This applies erratum 2701951 workaround to Cortex-A715