fix(cpus): workaround for Cortex-A76 erratum 2743102

Cortex-A76 erratum 2743102 is a Cat B erratum that applies to
all revisions <=r4p1 and is still open. The workaround is to
insert a dsb before the isb in the power down sequence.

SDEN documentation:
https://developer.arm.com/documentation/SDEN885749/latest



Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: Ie2cd73bd91417d30b5633d80b2fbee32944bc2de