iio: chemical: atlas-ph-sensor: rename atlas-ph-sensor to atlas-sensor

Since the orginal scope of the driver was to only support
the pH product from Atlas it has evolved to other sensors.

Rename the file, driver name, and regmap to atlas-sensor which
reflects this, although keep CONFIG_ATLAS_PH_SENSOR to not cause
regressions with current configurations.

Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>