ARM64: dts: meson-gx: add the missing uart_AO_B

This adds the missing node for the uart_AO_B port to the meson-gx.dtsi
(as this is supported by GXBB, GXL and GXM) along with the required
pinctrl pins. This is required as some boards are using it (the boards
from the Khadas VIM series for example have it exposed on the pin
headers).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>