fix(cpus): workaround for Cortex-A710 erratum 2768515

Cortex-A710 erratum 2768515 is a Cat B erratum that applies to
all revisions <=r2p1 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/SDEN1775101/latest



Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: If17fe04d3fda0dba6b8aabdd837a1c53e1830ed5