arm64: add read_cpuid_{implementor,part_number,mpidr}

In order to preserve some kind of source compatibility between
arm and arm64, introduce read_cpuid_{implementor,part_number,mpidr}
which are used on KVM to find out which CPU we're running on.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>