fix(cpus): workaround for Cortex-A77 erratum 2743100

Cortex-A77 erratum 2743100 is a Cat B erratum that applies to revisions
r0p0, r1p0, r1p1, and is still open. The workaround is to insert a dsb
before the isb in the power down sequence.

SDEN can be found here:
https://developer.arm.com/documentation/SDEN1152370/latest



Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I8e49a2dac8611f31ace249a17ae7a90cd60e742a