Workaround for Cortex A77 erratum 1925769

Cortex A77 erratum 1925769 is a Cat B erratum, present in older
revisions of the Cortex A77 processor core.  The workaround is to
set bit 8 in the ECTLR_EL1 register, there is a small performance cost
(<0.5%) for setting this bit.

SDEN can be found here:
https://documentation-service.arm.com/static/5f7c35d0d3be967f7be46d33

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: I9cf0e0b5dc1e3e32e24279d2632c759cc7bd7ce9
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index 8152c00..c976b8b 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -260,6 +260,9 @@
 -  ``ERRATA_A77_1800714``: This applies errata 1800714 workaround to Cortex-A77
    CPU. This needs to be enabled only for revision <= r1p1 of the CPU.
 
+-  ``ERRATA_A77_1925769``: This applies errata 1925769 workaround to Cortex-A77
+   CPU. This needs to be enabled only for revision <= r1p1 of the CPU.
+
 For Cortex-A78, the following errata build flags are defined :
 
 -  ``ERRATA_A78_1688305``: This applies errata 1688305 workaround to Cortex-A78