Remove redundantcy from the arm GIC softc

A struct recource already contains the bus_space_tag_t and
bus_space_handle_t. There is no neec to read them and store them again
in the drivers softc. Remove them and use the struct resource directly
with bus_read_* and bus_write_*.

Reviewed by: mmel
Differential Revision: https://reviews.freebsd.org/D28339