errata: workaround for Neoverse V1 errata 1774420

Neoverse V1 erratum 1774420 is a Cat B erratum present in r0p0 and
r1p0 of the V1 processor core. It is fixed in r1p1.

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

Signed-off-by: Lauren Wehrmeister <lauren.wehrmeister@arm.com>
Change-Id: I66e27b2518f73faeedd8615a1443a74b6a30f123
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index 949845a..a9d6e37 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -325,6 +325,10 @@
 
 For Neoverse V1, the following errata build flags are defined :
 
+-  ``ERRATA_V1_1774420``: This applies errata 1774420 workaround to Neoverse-V1
+   CPU. This needs to be enabled only for revisions r0p0 and r1p0, it is fixed
+   in r1p1.
+
 -  ``ERRATA_V1_1791573``: This applies errata 1791573 workaround to Neoverse-V1
    CPU. This needs to be enabled only for revisions r0p0 and r1p0, it is fixed
    in r1p1.