arm64: add MIDR_EL1 field accessors

The MIDR_EL1 register is composed of a number of bitfields, and uses of
the fields has so far involved open-coding of the shifts and masks
required.

This patch adds shifts and masks for each of the MIDR_EL1 subfields, and
also provides accessors built atop of these. Existing uses within
cputype.h are updated to use these accessors.

The read_cpuid_part_number macro is modified to return the extracted
bitfield rather than returning the value in-place with all other fields
(including revision) masked out, to better match the other accessors.
As the value is only used in comparison with the *_CPU_PART_* macros
which are similarly updated, and these values are never exposed to
userspace, this change should not affect any functionality.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>