iio: tsl2772: add support for reading proximity led settings from device tree

This patch adds support for optionally reading the proximity led diode
and current settings from device tree. This was tested using a LG
Nexus 5 (hammerhead) which requires a different diode than the driver
default for the IR LED.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>