staging:iio:ad7780: Remove the ad7780_platform_data

The ad7780_platform_data contains just the reference voltage information.
Since the preferred way of specifying this information is using the Linux
regulator framework and the ad7780 platform_data is not used by other
users, it can be completely removed.

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>