net: mdiobus: Introduce fwnode_mdbiobus_register()

This patch introduces a new helper function that
wraps acpi_/of_ mdiobus_register() and allows its
usage via common fwnode_ interface.

Fall back to raw mdiobus_register() in case CONFIG_FWNODE_MDIO
is not enabled, in order to satisfy compatibility
in all future user drivers.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>