iio: Add modifiers for ethanol and H2 gases

Add ethanol and H2 gas modifiers:
* IIO_MOD_ETHANOL
* IIO_MOD_H2

Signed-off-by: Andreas Brauchli <andreas.brauchli@sensirion.com>
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>