pinctrl: meson-axg: add missing GPIOA_18 gpio group

Without this, the gpio cannot be explicitly mux'ed to its gpio function.

Fixes: 83c56680

 ("pinctrl: meson-axg: Add new pinctrl driver for Meson AXG SoC")
Cc: stable@vger.kernel.org
Signed-off-by: Martin Hundebøll <martin@geanix.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Dmitry Rokosov <ddrokosov@sberdevices.ru>
Link: https://lore.kernel.org/r/20230512064925.133516-1-martin@geanix.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>