pinctrl: meson-axg: adjust uart_ao_b pin group naming

Simply adjust the pin group to _x _y _z style, as to
keep the consistency in DT with previous naming scheme.

Fixes: 83c56680 ("pinctrl: meson-axg: Add new pinctrl driver for Meson AXG SoC")
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>