staging: iio: isl29028: add runtime power management support

This patch adds runtime power management support to the isl29028 driver.
It defaults to powering off the device after two seconds of inactivity.

isl29028_chip_init_and_power_on() currently only zeros the CONFIGURE
register on the chip, which will cause the chip to turn off. This patch
also renames that function to isl29028_clear_configure_reg() since it is
now used in several places.

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