physmem ram: Don't reserve excluded regions

These regions can conflict with I/O resources and prevent allocation
of those regions by other drivers. It may make sense to reserve them
after the boot-time probe of devices has concluded (or after an
initial pass to reserve firmware-assigned resources before "wildcard"
resources are allocated), but that would require additional changes.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D43893