dt-bindings: serial: amlogic,meson-uart: Add compatible string for A9

dt-bindings: serial: amlogic,meson-uart: Add compatible string for A9

Amlogic A9 SoCs uses the same UART controller as S4 SoCs.
There is no need for an extra compatible line in the driver,
but add A9 compatible line for documentation.

Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Xianwei Zhao <xianwei.zhao@amlogic.com>
Link: https://patch.msgid.link/20260303-serial-binding-v1-1-c3df2a8f6fa3@amlogic.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>