Workaround for Cortex A76 erratum 1868343

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