regmap: mdio: Add clause-45 support

Modern ethernet phys support the so-called clause-45 register access
mode, which allows for register address widths of 16 bit.

Also allow for 16-bit register address widths, and return a regmap for
clause-45 access in that case.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Link: https://lore.kernel.org/r/9cc263e3e7d5865edd90453b4183f1cf363cb636.1622743333.git.sander@svanheule.net


Signed-off-by: Mark Brown <broonie@kernel.org>