staging: iio: adc: ad7192: Use DT clock binding

This patch replaces the platform data clock select member with DT clock
binding. Through the DT the external clock binding is specified. If this is
not provided then the device will use the internal clock source.

With the external clock binding there is the option to use a clock or a
crystal as the clock source. When an external crystal is used it is
connected to MCLK1 and MCLK2 pins. If the external clock is used only MCLK2
pin will be connected.

Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>