staging: iio: adc: ad7476 new SPI ADC driver

New driver handling:
	AD7475, AD7476, AD7477, AD7478, AD7466, AD7467, AD7468, AD7495
SPI micropower and high speed 12-/10-/8-Bit ADCs

staging: iio: adc: ad7476 apply list review feedback by Jonathan Cameron

Changes since last RFC post V1:
	Mainly list review feedback by Jonathan Cameron

-Remove scan_attrs from chip info structure.
-Remove name from chip info structure, use new spi_device_id instead.
-Allow transfer buffers to live in their own cache lines, to avoid DMA/cache coherency issues.
-Move scan el code into the ring buffer file.
-Use helper function to alloc the pollfunc.
-Use regulator framework and get vref_mv from the regulator in case not specified by pdata.
-Devices with buit-in reference use vref from the chip info structure
-Don't error on missing platform_data

-Make vref_mv type unsigned short
-Print in_scale "Vref / 2^(bits)" if fractional.

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>