fix(errata): workaround for Cortex-A78 erratum 2395406

Cortex-A78 erratum 2395406 is a cat B erratum that applies to revisions
r0p0 - r1p2 and is still open. The workaround is to set bit[40] of
CPUACTLR2 which will disable folding of demand requests into older
prefetches with L2 miss requests outstanding.

SDEN can be found here:
https://developer.arm.com/documentation/SDEN1401784



Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: If06f988f05f925c2a4bed3e6a9414b6acdfec894