arm64: Expose DC CVADP to userspace

ARMv8.5 builds upon the ARMv8.2 DC CVAP instruction by introducing a DC
CVADP instruction which cleans the data cache to the point of deep
persistence. Let's expose this support via the arm64 ELF hwcaps.

Signed-off-by: Andrew Murray <andrew.murray@arm.com>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>