miibus: Add support for mapping OFW nodes to PHY devices

Create a new miibus OFW specific layer leveraging miibus_fdt.c code.
PHY drivers can than read the properties using device_get_property(9) API.
Resource(interrupt) allocation is also supported.
In order to enable this each NIC/switch driver will have to be modified,
because of how miibus is attached to the parent driver.

Obtained from: Semihalf
Sponsored by: Alstom Group
Differential revision: https://reviews.freebsd.org/D32812