fix(errata): workaround for Cortex-X2 erratum 2147715

fix(errata): workaround for Cortex-X2 erratum 2147715

Cortex-X2 erratum 2147715 is a Cat B erratum that applies to revision
r2p0 and is fixed in r2p1. The workaround is to set CPUACTLR_EL1[22]=1,
which will cause the CFP instruction to invalidate all branch predictor
resources regardless of context.

SDEN can be found here:
https://developer.arm.com/documentation/SDEN1775100/latest

Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: I2d81867486d9130f2c36cd4554ca9a8f37254b57