pinctrl: amlogic: Add support for Amlogic Meson GXBB SoC

This patch adds the basic platform file to support the pin controller
found on the Amlogic Meson GXBB SoCs.

Signed-off-by: Carlo Caione <carlo@endlessm.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>