pinctrl: add compatible for Amlogic Meson A1 pin controller

Add new compatible name for Amlogic's Meson-A1 pin controller
add a dt-binding header file which document the detail pin names.
Note that A1 doesn't need DS bank reg any more, use gpio reg as
base.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Qianggui Song <qianggui.song@amlogic.com>
Link: https://lore.kernel.org/r/1572004167-24150-2-git-send-email-qianggui.song@amlogic.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>