fix(cpus): workaround for Neoverse-N2 erratum 2388450

Neoverse-N2 erratum 2388450 is a cat B erratum that applies to
revision r0p0 and is fixed in r0p1. The workaround is to set
bit[40] of CPUACTLR2_EL1 to disable folding of demand requests into
older prefetches with L2 miss requests outstanding.

SDEN can be found here:
https://developer.arm.com/documentation/SDEN1982442/latest



Change-Id: I6dd949c79cea8dbad322e569aa5de86cf8cf9639
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>