Workaround for Cortex A76 erratum 1946160

Workaround for Cortex A76 erratum 1946160

Cortex A76 erratum 1946160 is a Cat B erratum, present in some revisions
of the A76 processor core.  The workaround is to insert a DMB ST before
acquire atomic instructions without release semantics.  This issue is
present in revisions r0p0 - r4p1  but this workaround only applies to
revisions r3p0 - r4p1, there is no workaround for older versions.

SDEN can be found here:
https://documentation-service.arm.com/static/5fbb77d7d77dd807b9a80cc1

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: Ief33779ee76a89ce2649812ae5214b86a139e327