fix(cpus): workaround for Cortex-A78 erratum 2772019

Cortex-A78 erratum 2772019 is a Cat B erratum that applies to
all revisions <=r1p2 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/SDEN1401784/latest



Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: I0362da463eca777aa7a385bcdeb39b8549799f02