iio: adc: imx93: add four channels for imx93 adc

According to the spec, this ADC totally support 8 channels.
i.MX93 contain this ADC with 4 channels connected to pins in
the package. i.MX95 contain this ADC with 8 channels connected
to pins in the package.

Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Fixes: 7d02296a

 ("iio: adc: add imx93 adc support")
Link: https://lore.kernel.org/r/20231116071026.611269-1-haibo.chen@nxp.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>