arm64: Expose Arm v8.4 features

Expose the new features introduced by Arm v8.4 extensions to
Arm v8-A profile.

These include :

 1) Data indpendent timing of instructions. (DIT, exposed as HWCAP_DIT)
 2) Unaligned atomic instructions and Single-copy atomicity of loads
    and stores. (AT, expose as HWCAP_USCAT)
 3) LDAPR and STLR instructions with immediate offsets (extension to
    LRCPC, exposed as HWCAP_ILRCPC)
 4) Flag manipulation instructions (TS, exposed as HWCAP_FLAGM).

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