iio: adc: New driver for Cirrus Logic EP93xx ADC

New driver adding support for ADC found on Cirrus Logic EP93xx series of SoCs.
Board specific code must take care to create plaform device with all necessary
resources.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>