iio: hid: Add temperature sensor support

Environmental temperature sensor is a hid defined sensor,
it measures temperature.

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

According to IIO ABI definition, IIO_TEMP data output unit is
milli degrees Celsius. Add the unit convert from degree to milli degree.

Signed-off-by: Song Hongyan <hongyan.song@intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>