bus_if: Add a default implementation of get_property

There are multiple buses that pretend to be ofw compatible,
e.g ofw_pci, mii_fdt. We now need to provide an implementation
of BUS_GET_PROPERTY for every one of them. Instead of modifying
them one by one it's better to just provide a default
implementation that simply traverses up the device tree.
Remove the now unneeded BUS_GET_PROPERTY implementation in mii_fdt.

Reviewed by: andrew, bz
Obtained from: Semihalf
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D34031