iio: health: Add driver for the TI AFE4403 heart monitor

Add driver for the TI AFE4403 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.

Data sheet located here:
http://www.ti.com/product/AFE4403/datasheet



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