fix(cpus): workaround for Cortex-A78C erratum 2772121

fix(cpus): workaround for Cortex-A78C erratum 2772121

Cortex-A78C erratum 2772121 is a Cat B erratum that applies to
all revisions <=r0p2 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/SDEN1707916/latest

Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: I0e190dabffc20c4d3b9b98d1abeb50f308b80bb9