staging: IIO: ADC: New driver for the AD7780 / AD7781 24-bit Sigma-Delta ADC

Changes since v1:

IIO: ADC: AD7780: Apply review feedback

Abort probe() with error in case platform_data is not present.
Add comment explaining power down reset (PDRST) GPIO handling
and dual function DOUT/RDY interrupt usage.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>