arm64: kernel: add MPIDR_EL1 accessors macros

In order to simplify access to different affinity levels within the
MPIDR_EL1 register values, this patch implements some preprocessor
macros that allow to retrieve the MPIDR_EL1 affinity level value according
to the level passed as input parameter.

Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>