pinctrl: Add compatibles for Amlogic Meson G12A pin controllers

Add new compatible name for Amlogic's Meson-G12A pin controllers,
add a dt-binding header file which document the detail pin names.

Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Xingyu Chen <xingyu.chen@amlogic.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>