arm64: cpufeature: Reduce number of pointer auth CPU caps from 6 to 4

We can easily avoid defining the two meta-capabilities for the address
and generic keys, so remove them and instead just check both of the
architected and impdef capabilities when determining the level of system
support.

Reviewed-by: Suzuki Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>