staging:iio:ad7780: Make powerdown GPIO optional

Some designs hardwire the PDRST pin to always on. In this case there is no GPIO
to control the mode of the device, so make the GPIO optional. Since now all of
the the platform data fields are optional now, make the platform data as a whole
optional as well.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>