Introduce ofw_bus_reg_to_rl() to replace part of common bus code

Instead of reusing the same reg parsing code, create one, common function
that puts reg contents to the resource list. Address cells and size cells
are passed rather than acquired here so that any bus can have different
default values.

Obtained from: Semihalf
Reviewed by: andrew, ian, nwhitehorn
Sponsored by: The FreeBSD Foundation