iio: hid: Add humidity sensor support

Environmental humidity sensor is a hid defined sensor,
it shows raw humidity measurement of air.

More information can be found in:
http://www.usb.org/developers/hidpage/HUTRR39b.pdf

According to IIO ABI definition, humidityrelative data output unit is
milli percent. Add the unit convert from percent to milli percent.

Signed-off-by: Song Hongyan <hongyan.song@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>