iio: adc: rockchip_saradc: add support for rk3568 saradc

It is similar to other devices, but with 8 channels.

Signed-off-by: Simon Xue <xxm@rock-chips.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20210712014507.97477-1-xxm@rock-chips.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>