staging: IIO: ADC: New driver for AD7606/AD7606-6/AD7606-4

This patch adds support for the:
AD7606/AD7606-6/AD7606-4 8/6/4-Channel Data Acquisition
system (DAS) with 16-Bit, Bipolar, Simultaneous Sampling ADC.

Changes since V1:
IIO: ADC: New driver for AD7606/AD7606-6/AD7606-4: Apply review feedback

Rename sysfs node oversampling to oversampling_ratio.
Kconfig: Add GPIOLIB dependency.
Use range in mV to better match HWMON.
Rename ad7606_check_oversampling.
Fix various comments and style.
Reorder is_visible cases.
Use new gpio_request_one/array and friends.
Drop check for SPI max_speed_hz.

Changes since V2:
IIO: ADC: New driver for AD7606/AD7606-6/AD7606-4: Apply review feedback

Documentation: specify unit
Avoid raise condition in ad7606_scan_direct()
Check return value of bus ops read_block()

Changes since V3:
IIO: ADC: New driver for AD7606/AD7606-6/AD7606-4: Add missing include file

Add linux/sched.h

Changes since V4:
IIO: ADC: New driver for AD7606/AD7606-6/AD7606-4: Fix kconfig declaration

consistently use tristate to avoid configuration mismatches

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>