arm64: dts: meson: g12a: Add UART A, B & C nodes and pins

This patch adds the 3 UART nodes in the EE power domain with the corresponding
pinctrl nodes.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>