iio: adf4350: Convert to use GPIO descriptor

The lock detect GPIO line is better to grab using
a GPIO descriptor. We drop the pdata for this: clients using board
files can use machine descriptor tables to pass this GPIO from
static data.

Cc: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>