arm64: dts: meson-axg: switch uart_ao clock to CLK81

Switch the uart_ao pclk to CLK81 since the clock driver is ready.

Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>