fix(cpus): workaround for Neoverse N2 erratum 2743089

Neoverse N2 erratum 2743089 is a Cat B erratum that applies to
all revisions <=r0p2 and is fixed in r0p3. The workaround is to
insert a dsb before the isb in the power down sequence.

SDEN documentation:
https://developer.arm.com/documentation/SDEN1982442/latest



Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: Idec862226bd32c91374a8bbd5d73d7ee480a34d9