dt-bindings: iio: adc: amlogic,meson-saradc: add S4 compatible

Add the compatible string for the SARADC (Successive Approximation
Register ADC) IP block found in the Amlogic Meson S4 SoC.

There are no known differences between the SARADC on S4 and the one
on G12A. Therefore, it uses "amlogic,meson-g12a-saradc" as a proper
specific fallback.

Also add a comment indicating that "amlogic,meson-saradc" must not be
used for new devices. It's a made up compatible string that does not
correspond to a specific hardware generation and is not used to match
any driver. For old devices we keep it as it's part of the ABI.

Signed-off-by: Nick Xie <nick@khadas.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>