staging:iio:ad7606: Add support for the ad7605-4

Add support for the AD7605-4 to the AD7606 driver. The AD7605-4 is mostly
interface compatible to the AD7606-6 with the only difference being not
having support for oversampling.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>