ARM64: dts: meson-axg: uart: Add the pinctrl info description

Describe the pinctrl info for the UART controller which is found
in the Meson-AXG SoCs.

Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
[khilman: s/uart_ao_b_gpioz/uart_ao_b_z/ ]
Signed-off-by: Kevin Hilman <khilman@baylibre.com>