arm64: dts: meson-g12-common: add uart_ao_b pins muxing

- RX/TX signals can be mapped on 2 different pairs of pins so supporting
  both options
- RTS/CTS signals however only have 1 option available

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lore.kernel.org/r/20220112211642.2248901-4-gary.bisson@boundarydevices.com