arm64: cpufeature: Define helpers for sys_reg id

Define helper macros to extract op0, op1, CRn, CRm & op2
for a given sys_reg id. While at it remove the explicit
masking only used for Op0.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>