iio: health: Add driver for the TI AFE4404 heart monitor

Add driver for the TI AFE4404 heart rate monitor and pulse oximeter.
This device detects reflected LED light fluctuations and presents an ADC
value to the user space for further signal processing.

Datasheet: http://www.ti.com/product/AFE4404/datasheet



Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>