fix(cpus): workaround for Cortex-X2 erratum 2742423

Cortex-X2 erratum 2742423 is a Cat B erratum that applies to all
revisions <= r2p1 and is still open.
The workaround is to set CPUACTLR5_EL1[56:55] to 2'b01.

SDEN documentation:
https://developer.arm.com/documentation/SDEN-1775100/latest



Change-Id: I03897dc2a7f908937612c2b66ce7a043c1b7575d
Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>