errata: workaround for Neoverse V1 errata 2139242

Neoverse V1 erratum 2139242 is a Cat B erratum present in the V1
processor core.  This issue is present in revisions r0p0, r1p0,
and r1p1, and it is still open.

SDEN can be found here:
https://documentation-service.arm.com/static/60d499080320e92fa40b4625

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: I5c2e9beec72a64ac4131fb6dd76199821a934ebe
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index 4d7b179..bc277a7 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -349,6 +349,10 @@
    issue is present in r0p0 as well but there is no workaround for that
    revision.  It is still open.
 
+-  ``ERRATA_V1_2139242``: This applies errata 2139242 workaround to Neoverse-V1
+   CPU. This needs to be enabled for revisions r0p0, r1p0, and r1p1 of the
+   CPU.  It is still open.
+
 DSU Errata Workarounds
 ----------------------