mdio: introduce Clause 45 APIs

Add `mdio_read_c45()`/`mdio_write_c45()` APIs for Clause 45 access
and remove the `protocol` MDIO binding property so that MDIO bus
controller can support more than one protocol.

A new MDIO header is introduced with generic opcodes, MMD and
registers addresses, to be used by MDIO and PHY drivers.

Existing MDIO drivers that support both Clause 22 and Clause 45
access are migrated to the new APIs.

Signed-off-by: Manuel Argüelles <manuel.arguelles@nxp.com>