staging: iio: ad9832: use clock framework for clock reference

Previously external clock were set through platform_data struct.
Now device uses clk struct defined in include/linux/clk.h to handle
external clock source.
It also removes mclk from platform_data struct.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>