fix(errata): workaround for Cortex-A510 erratum 2971420

Cortex-A510 erratum 2971420 applies to revisions r0p1, r0p2, r0p3,
r1p0, r1p1, r1p2 and r1p3, and is still open.

Under some conditions, data might be corrupted if Trace Buffer
Extension (TRBE) is enabled. The workaround is to disable trace
collection via TRBE by programming MDCR_EL3.NSTB[1] to the opposite
value of SCR_EL3.NS on a security state switch. Since we only enable
TRBE for non-secure world, the workaround is to disable TRBE by
setting the NSTB field to 00 so accesses are trapped to EL3 and
secure state owns the buffer.

SDEN: https://developer.arm.com/documentation/SDEN-1873361/latest/

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: Ia77051f6b64c726a8c50596c78f220d323ab7d97
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index 9e06fe0..6562139 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -956,6 +956,10 @@
    Cortex-A510 CPU. This needs to be applied to revision r0p0, r0p1, r0p2,
    r0p3, r1p0, r1p1 and r1p2. It is fixed in r1p3.
 
+-  ``ERRATA_A510_2971420``: This applies erratum 2971420 workaround to
+   Cortex-A510 CPU. This needs to be applied to revisions r0p1, r0p2, r0p3,
+   r1p0, r1p1, r1p2 and r1p3 and is still open.
+
 For Cortex-A520, the following errata build flags are defined :
 
 -  ``ERRATA_A520_2630792``: This applies errata 2630792 workaround to