fix(cpus): workaround for Cortex-A78 erratum 2742426

Cortex-A78 erratum 2742426 is a Cat B erratum that applies to
all revisions <= r1p2 and is still open.

The workaround is to set the CPUACTLR5_EL1[56:55] to 2'b01.

SDEN documentation:
https://developer.arm.com/documentation/SDEN1401784/latest



Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: I42506a87d41c9e2b30bc78c08d22f36e1f9635c1