iio: humidity: hdc3020: Add Makefile, Kconfig and MAINTAINERS entry

Something when wrong when applying the original patch and only the
c file made it in.  Here the rest of the changes are applied.

Fixes: c9180b8e

 ("iio: humidity: Add driver for ti HDC302x humidity sensors")
Reported-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Cc: Li peiyu <579lpy@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>