fix(cpus): workaround for Neoverse N1 erratum 2743102

Neoverse N1 erratum 2743102 is a Cat B erratum that applies to
all revisions <=r4p1 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/SDEN885747/latest



Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: I81a8793c1a118764df3ac97b67f5e088f56f6a20