ARM64: dts: meson-axg: enable the UART_A controller

The UART_A is connected to a BT module on the S400 board.

Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>