gic_v3: consolidate per-cpu redistributor information

Update 'struct gic_redists' to consolidate all per-cpu redistributor
information into a new 'struct redist_pcpu'. Provide a new interface
(GICV3_IVAR_REDIST) for the GIC driver, which can be used to retrieve
the per-cpu data.

This per-cpu redistributor struct will be later used to improve the
GIC ITS setup.

While there, remove some unused fields in gic_v3_var.h interface.
No functional changes.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D19842