bhnd_chipc: Make use of bus_generic_rman_* to simplify some code

This uses bus_generic_rman_alloc/release_resource to reduce some code
duplication. However, I've left the custom activate/deactivate
methods as-is.

Differential Revision: https://reviews.freebsd.org/D43939

(cherry picked from commit 3a48dfe115f35abac6c2d9ec03a6a574baeaf0bc)