regmap: Add MDIO bus support

Basic support for MDIO bus access. Support only includes clause-22
register access, with 5-bit addresses, and 16-bit wide registers.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Link: https://lore.kernel.org/r/63b99a2fec2c4ea3c461d59d451af8d675ecf312.1621279162.git.sander@svanheule.net


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