mdio_bus: Simplify reset handling and extend to non-DT systems

Convert mdiobus_register_reset() from open-coded DT-only optional reset
handling to reset_control_get_optional_exclusive().  This not only
simplifies the code, but also adds support for lookup-based resets on
non-DT systems.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>