iio: adc: New driver for TI ADS7950 chips

This adds a new driver for the TI ADS7950 family of ADC chips. These
communicate using SPI and come in 8/10/12-bit and 4/8/12/16 channel
varieties.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>