pinctrl: Add Microsemi Ocelot SoC driver

The Microsemi Ocelot SoC has a few pins that can be used as GPIOs or take
multiple other functions. Add a driver for the pinmuxing and the GPIOs.

There is currently no support for interrupts.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>