staging:iio:ad7780: Switch to the gpio descriptor interface

Use the gpiod interface for the powerdown_gpio instead of the deprecated
old non-descriptor interface.

The powerdown pin can be tied high, so the gpio is optional.
Remove the gpio_pdrst platform_data member since the new interface is
used.

Signed-off-by: Dragos Bogdan <dragos.bogdan@analog.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>