irqchip/gic-v3: Improve affinity helper

The GICv3 driver uses multiple formats for the affinity, all
derived from a reading of MPDR_EL1 on one CPU or another.

Simplify the handling of these affinity by moving the access
to the CPU affinity via cpu_logical_map() inside the helper,
and rename it accordingly.

This will be helpful to support some more broken hardware.

Signed-off-by: Marc Zyngier <maz@kernel.org>