iio: Add tcs3472 color light sensor driver

chip has four 16-bit channels for red, green, blue, clear color
intensity; driver supports the TCS3x7x family of devices and was
tested with a TCS34725 chip; further information here:
http://www.ams.com/eng/Products/Light-Sensors/Color-Sensor/TCS34725



v2 (thanks to Jonathan Cameron):
* drop dynamic buffer allocation, buffer is in tcs3472_data
* limit sysfs output to PAGE_SIZE
* check val2 == 0 when writing CALIBSCALE

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Cc: Jon Brenner <jon.brenner@ams.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>