iio: adc: meson: add support for the GXLX SoC

iio: adc: meson: add support for the GXLX SoC

The SARADC IP on the GXLX SoC itself is identical to the one found on
GXL SoCs. However, GXLX SoCs require poking the first three bits in the
MESON_SAR_ADC_REG12 register to get the three MPLL clocks (used as clock
generators for the audio frequencies) to work.

The reason why there are MPLL clock bits in the ADC register space is
entirely unknown and it seems that nobody is able to comment on this.
So clearly mark this as a workaround and add a warning so users are
notified that this workaround can change (once we know what these bits
actually do).

Tested-by: Christian Hewitt <christianshewitt@gmail.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://patch.msgid.link/20250330101922.1942169-3-martin.blumenstingl@googlemail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>