iio: health: max30100: add config for LED current

Allow the current for both RED and IR LEDs to be set via an device tree
property setting.

This is an optional setting that is useful for applications that have a
known glass attenuation factor.

Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>