iio: adc: meson-saradc: add support for Meson S4

iio: adc: meson-saradc: add support for Meson S4

Add support for the SARADC found on the Amlogic Meson S4 SoC.
According to the documentation and current testing, it is fully
compatible with the G12A parameter set, so we reuse
`meson_sar_adc_g12a_data` for this new compatible string.

Although the device tree fallback mechanism could handle the match,
a dedicated entry is added to ensure the userspace ABI correctly
reports the specific part name ("meson-s4-saradc"). This allows
userspace to accurately identify the exact device and maintains
consistency across different firmware types where automatic fallback
parsing might be problematic.

Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Nick Xie <nick@khadas.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>