iio: tsl2583: add runtime power management support

This patch adds runtime power management support to the tsl2583 driver.
The device is powered off after two seconds of inactivity. Verified that
the driver still functions correctly using a TSL2581 hooked up to a
Raspberry Pi 2.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>