iio: ak8975: Convert to use GPIO descriptor

The end-of-conversion (EOC) 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: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>