Implement support for RF_UNMAPPED and bus_map/unmap_resource on x86.

Add implementations of bus_map/unmap_resource to the x86 nexus driver.
Change bus_activate/deactivate_resource to honor RF_UNMAPPED and to
use bus_map/unmap_resource to create/destroy the implicit mapping when
RF_UNMAPPED is not set.

Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D5237