fix(errata): workaround for Cortex-A710 erratum 2008768

Cortex-A710 erratum 2008768 is a Cat B erratum that applies to revisions
r0p0, r1p0, and r2p0, and is fixed in r2p1. The workaround is to clear
the ED bit in each ERXCTLR_EL1 register before setting the PWRDN bit in
CPUPWRCTLR_EL1.

SDEN can be found here:
https://developer.arm.com/documentation/SDEN1775101



Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: Ib2171c06da762dd4155b02c03d86766f1616381d