staging: iio: light sensor: Add a calibscale file to the isl29018 light sensor driver.

Defaulting to 1, this gives a way to amplify the lux value before being
reduced by the programmed adc_bit shift.
Only support whole numbers right now.  When this driver is converted to the new
IIO_CHAN framework, it will be easy to support the framework's pseudo float.

Add illuminance0_calibscale documentation to sysfs-bus-iio-light.

Signed-off-by: Bryan Freed <bfreed@chromium.org>
Acked-by: Rhyland Klein <rklein@nvidia.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>