dt-bindings: iio: adc: ad7192: Add clock provider

dt-bindings: iio: adc: ad7192: Add clock provider

Internal clock of AD719X devices can be made available on MCLK2 pin. Add
clock provider to support this functionality when clock cells property
is present.

The clock source can be either provided externally or the internal clock
is used. Pair of clocks and clock-names property is mutally exclusive
with #clock-cells property.

Modify second example to showcase the mode where internal clock is used.

Signed-off-by: Alisa-Dariana Roman <alisa.roman@analog.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20240717212535.8348-4-alisa.roman@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>