pinctrl: add driver for Amlogic Meson SoCs

This is a driver for the pinmux and GPIO controller available in
Amlogic Meson SoCs. It currently supports only Meson8, however the
common code should be generic enough to work also for other SoCs after
having defined the proper set of functions and groups.

GPIO interrupts are not supported at the moment due to lack of
documentation.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>