staging: iio: ad5933: use clock framework for clock reference

Add the option to specify the external clock (MCLK) using the clock
framework.
Also remove the old platform data structure.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Signed-off-by: Gabriel Capella <gabriel@capella.pro>
Co-developed-by: Gabriel Capella <gabriel@capella.pro>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>