fix(cpus): workaround for Cortex-X3 erratum 2313909

fix(cpus): workaround for Cortex-X3 erratum 2313909

Cortex-X3 erratum 2313909 is a Cat B erratum that applies to revisions
r0p0 and r1p0, and is fixed in r1p1. The workaround is to set
CPUACTLR2_EL1[36] to 1 before the power down sequence that sets
CORE_PWRDN_EN. This allows the cpu to retry the power down and prevents
the deadlock. TF-A never clears this bit even if it wakes up from the
wfi in the sequence since it is not expected to do anything but retry to
power down after and the bit is cleared on reset.

SDEN can be found here:
https://developer.arm.com/documentation/SDEN2055130/latest

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I5935b4bcd1e6712477c0d6eab2acc96d7964a35d