iio: adc: Support ROHM BD79112 ADC/GPIO

iio: adc: Support ROHM BD79112 ADC/GPIO

The ROHM BD79112 is an ADC/GPIO with 32 channels. The channel inputs can
be used as ADC or GPIO. Using the GPIOs as IRQ sources isn't supported.

The ADC is 12-bit, supporting input voltages up to 5.7V, and separate I/O
voltage supply. Maximum SPI clock rate is 20 MHz (10 MHz with
daisy-chain configuration) and maximum sampling rate is 1MSPS.

The IC does also support CRC but it is not implemented in the driver.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>