iio: chemical: add atlas-ezo-sensor initial support

Add driver for Atlas EZO line of sensors with initial support for
CO2 the sensor. This is effectively ASCII strings proxied over I2C
due to these series of sensors being by default UART.

Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>