iio: adc: Initial support for AD4134

iio: adc: Initial support for AD4134

AD4134 is a 24-bit, 4-channel, simultaneous sampling, precision
analog-to-digital converter (ADC). The device can be managed through SPI or
direct control of pin logical levels (pin control mode). The AD4134 design
also features a dedicated bus for ADC sample data output. Though, this
initial driver for AD4134 only supports usual SPI connections.

Add basic support for AD4134 that enables single-shot ADC sample read.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>