dt-bindings: iio: adc: amlogic,meson-saradc: Add GXLX SoC compatible

Add a compatible string for the GXLX SoC. It's very similar to GXL but
has three additional bits in MESON_SAR_ADC_REG12 for the three MPLL
clocks.

Acked-by: Krzysztof Kozlowski <[e-mail]>
Reviewed-by: Neil Armstrong <[e-mail]>
Signed-off-by: Martin Blumenstingl <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/devicetree/bindings/iio/adc/amlogic,meson-saradc.yaml

···
               - amlogic,meson8m2-saradc
               - amlogic,meson-gxbb-saradc
               - amlogic,meson-gxl-saradc
+              - amlogic,meson-gxlx-saradc
               - amlogic,meson-gxm-saradc
               - amlogic,meson-axg-saradc
               - amlogic,meson-g12a-saradc