fix(cpus): workaround for Cortex-X2 erratum 2768515

fix(cpus): workaround for Cortex-X2 erratum 2768515

Cortex-X2 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/SDEN1775100/latest

Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: Ib02688f7b6dc7f6ec305e68e8895174f6fd577a0