Workaround for Cortex A76 erratum 1868343

Cortex A76 erratum 1868343 is a Cat B erratum, present in older
revisions of the Cortex A76 processor core.  The workaround is to
set a bit in the CPUACTLR_EL1 system register, which delays instruction
fetch after branch misprediction. This workaround will have a small
impact on performance.

This workaround is the same as workarounds for errata 1262606 and
1275112, so all 3 have been combined into one function call.

SDEN can be found here:
https://documentation-service.arm.com/static/5f2bed6d60a93e65927bc8e7

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: I7f2f9965f495540a1f84bb7dcc28aff45d6cee5d
diff --git a/docs/design/cpu-specific-build-macros.rst b/docs/design/cpu-specific-build-macros.rst
index e9ff17e..8152c00 100644
--- a/docs/design/cpu-specific-build-macros.rst
+++ b/docs/design/cpu-specific-build-macros.rst
@@ -249,6 +249,9 @@
    limitation of errata framework this errata is applied to all revisions
    of Cortex-A76 CPU.
 
+-  ``ERRATA_A76_1868343``: This applies errata 1868343 workaround to Cortex-A76
+   CPU. This needs to be enabled only for revision <= r4p0 of the CPU.
+
 For Cortex-A77, the following errata build flags are defined :
 
 -  ``ERRATA_A77_1508412``: This applies errata 1508412 workaround to Cortex-A77