staging: iio: frequency: ad9833: Load clock using clock framework

The clock frequency is loaded from device-tree using clock framework
instead of statically value. The change allow configuration of
the device via device-trees and better initialization sequence.
This is part of broader effort to add device-tree support to this driver
and take it out from staging.

Signed-off-by: Beniamin Bia <beniamin.bia@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>