IIO: ADC: New driver for the AD7298 8-channel SPI ADC

This patch adds support for the
AD7298:  8-Channel, 1MSPS, 12-Bit SAR ADC with Temperature Sensor
via SPI bus.

This patch replaces the existing ad7298.c driver completely.
It was necessary since, the old driver did not comply with the
IIO ABI for such devices.

Changes since V1:
IIO: ADC: New driver for the AD7298 8-channel SPI ADC

Add documentation for new sysfs file tempX_input.
Simplify bit defines.
Remove outdated comments.
Fix indention style.
ad7298_show_temp():
	Add locking.
	Simplify temperature calculation.
	Change temperature result from degrees into milli degrees Celsius.
	Rename file according to new sysfs ABI documentation

Add timestamp attributes.
Revise timestamp handling accordingly.
Preset timestamp generation.

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