ARM: Use implementor and part defines from cputype.h

Instead of decoding implementor numbers, part numbers and Xscale
architecture masks inline in the pmu probing function, use defines
and accessor functions from cputype.h, which can also be shared by
other subsystems, such as KVM.

Signed-off-by: Christoffer Dall <c.dall@virtualopensystems.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>