xlat v2: Dynamically detect need for CnP bit

ARMv8.2-TTCNP is mandatory from ARMv8.2 onwards, but it can be implemented
in CPUs that don't implement all mandatory 8.2 features (and so have to
claim to be a lower version).

This patch removes usage of the ARM_ARCH_AT_LEAST() macro and uses system
ID registers to detect whether it is needed to set the bit or not.

Change-Id: I7bcbf0c7c937590dfc2ca668cfd9267c50f7d52c
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>