iio: humidity: hts221: add regmap API support

Introduce regmap API support to access to i2c/spi bus instead of
using a custom support.
Remove lock mutex since concurrency is already managed by regmap API

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>