fix(errata): workaround for Cortex A78 AE erratum 2376748

Cortex A78 AE erratum 2376748 is a Cat B erratum that applies
to revisions <= r0p1. It is still open.

The erratum states, "A PE executing a PLDW or PRFM PST instruction
that lies on a mispredicted branch path might cause a second PE
executing a store exclusive to the same cache line address to fail
continuously."

The erratum is avoided by setting CPUACTLR2_EL1[0] to 1 to force
PLDW/PFRM ST to behave like PLD/PRFM LD and not cause invalidations
to other PE caches. There might be a small performance degradation
to this workaround for certain workloads that share data.

SDEN is available at https://developer.arm.com/documentation/SDEN-1707912



Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>
Change-Id: I93bd392a870d4584f3e12c8e4626dbe5a3a40a4d