realtek: add switch core pinctrl driver

realtek: add switch core pinctrl driver

Since control bits for different pin muxes are scattered throughout the
switch core's register space, pin control features are provided by a
table of regmap fields. This should be flexible enough to allow support
for currently unsupported pin muxes, and newer SoC generations providing
similar features.

This driver add initial support for Maple and Cypress managed gigabit
switch SoCs, with RTL838xM and RTL839xM part numbers. The fast ethernet
switch SoCs from these parts series, with numbers RTL833xM and RTL835xM,
should also be supported, but have not been tested.

Signed-off-by: Sander Vanheule <sander@svanheule.net>