dt-bindings: serial: amlogic,meson-uart: Add compatible string for A4

Amlogic A4 SoCs uses the same UART controller as S4 SoCs and G12A.
There is no need for an extra compatible line in the driver, but
add A4 compatible line for documentation.

Signed-off-by: Xianwei Zhao <xianwei.zhao@amlogic.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://lore.kernel.org/r/20240401-basic_dt-v3-3-cb29ae1c16da@amlogic.com


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>