IIO: ADC: New driver for AD7190/AD7192/AD7195 4 Channel SPI ADC

New driver for AD7190/AD7192/AD7195 4.8 kHz, Ultralow Noise, 24-Bit
Sigma-Delta ADC with PGA

These devices features a dual use data out ready DOUT/RDY output.
In order to avoid contentions on the SPI bus, it's necessary to use
spi bus locking. The DOUT/RDY output must also be wired to an
interrupt capable GPIO.

In INDIO_RING_TRIGGERED mode, this driver may block its SPI bus segment
for an extended period of time.

Changes since V1:

Add missing documentation.
Remove obsoleted include files.
Fix typos and style issues.
Fix buffer size.
Split ad7192_show() into two functions.
Avoid race condition add mutex.
Abandon IIO_CHAN macro.
Reorder elements in ad7192_platform_data.
Remove driver bus type.

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>