iio: core: Add IIO_COVERAGE channel type

iio: core: Add IIO_COVERAGE channel type

Add a new channel type for sensors that report fractional coverage as
a percentage. The sysfs attribute is in_coverageY_raw; after applying
in_coverageY_scale the value is in percent. The first user is the
ADT7604 leak detector, where the value represents the portion of the
sensing element that is wetted.

Signed-off-by: Liviu Stan <liviu.stan@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>