regmap: Rework regmap_mdio_c45_{read|write} for new C45 API.

The MDIO subsystem is getting rid of MII_ADDR_C45 and thus also
encoding associated encoding of the C45 device address and register
address into one value. regmap-mdio also uses this encoding for the
C45 bus.

Move to the new C45 helpers for MDIO access and provide regmap-mdio
helper macros.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20230116111509.4086236-1-michael@walle.cc
Signed-off-by: Mark Brown <broonie@kernel.org>