dt-bindings: rtc: amlogic,a4-rtc: Add compatible string for C3

Amlogic C3 SoCs uses the same rtc controller as A5 SoCs. There is
no need for an extra compatible line in the driver, but add C3
compatible line for documentation.

Signed-off-by: Xianwei Zhao <xianwei.zhao@amlogic.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250717-rtc-c3-node-v1-1-4f9ae059b8e6@amlogic.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>