iio: Add Lite-On ltr501 ambient light / proximity sensor driver

iio: Add Lite-On ltr501 ambient light / proximity sensor driver

combined ambient light (two channels) and proximity sensor with I2C
interface; the ALS channels are visible+IR and IR

datasheet is here
http://optoelectronics.liteon.com/upload/download/DS86-2012-0006/P_100_LTR-501ALS-01_PrelimDS_ver1.1.pdf

v3:
* fix use of sizeof in _read_als()
v2: (thanks to Lars-Peter Clausen)
* cannot use devm_iio_device_register() due to cleanup order in _remove()
* mutex around data wait/read
* turn info message in _probe() into check for part number
* change copyright year to 2014

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>