pinctrl: mscc: Add gpio and pinctrl driver for MSCC MIPS SoCs (VcoreIII based)

This driver supports the pin and gpio controller found in the Ocelot and
Luton SoCs.

The driver was inspired from the pinctrl driver in Linux, but was
simplified and was modified to allow supporting an other SoCs (Luton).

For Ocelot and Luton the controller is the same, only the pins to program
differ.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
[changed to only descend into mscc/ dependent on CONFIG_PINCTRL_MSCC]
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>