fix(errata): workaround for Cortex-A510 erratum 2042739

Cortex-A510 erratum 2042739 is a Cat B erratum that applies to revisions
r0p0, r0p1 and r0p2 and is fixed in r0p3.

SDEN can be found here:
https://developer.arm.com/documentation/SDEN2397239

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: I1d2ebee3914396e1e298eb45bdab35ce9e194ad9
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index 7ba5023..95d43c4 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -484,6 +484,10 @@
    Cortex-A510 CPU. This needs to be enabled only for revisions r0p0, r0p1,
    r0p2, r0p3 and r1p0, it is fixed in r1p1.
 
+-  ``ERRATA_A510_2042739``: This applies errata 2042739 workaround to
+   Cortex-A510 CPU. This needs to be enabled only for revisions r0p0, r0p1 and
+   r0p2, it is fixed in r0p3.
+
 DSU Errata Workarounds
 ----------------------