fix(cpus): workaround for Cortex-X2 erratum 2778471

Cortex-X2 erratum 2778471 is a Cat B erratum that applies
to revisions r0p1, r1p0, r2p0 and r2p1 and is still open.
The workaround is to set CPUACTLR3_EL1[47] to 1.

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



Change-Id: Ia95f0e276482283bf50e06c58c2bc5faab3f62c6
Signed-off-by: Sona Mathew <sonarebecca.mathew@arm.com>