fix(cpus): workaround for Cortex-X3 erratum 2615812

fix(cpus): workaround for Cortex-X3 erratum 2615812

Cortex-X3 erratum 2615812 is a Cat B erratum that applies to revisions
r0p0, r1p0, and r1p1, and is still open. The workaround is to disable
the use of the Full Retention power mode in the core (setting
WFI_RET_CTRL and WFE_RET_CTRL in CORTEX_X3_IMP_CPUPWRCTLR_EL1 to 0b000).

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

Change-Id: I5ad66df3e18fc85a6b23f6662239494ee001d82f
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>