dax: Document struct dev_dax_range

dax: Document struct dev_dax_range

The device DAX structure is being enhanced to track additional DCD
information.  Specifically the range tuple needs additional parameters.
The current range tuple is not fully documented and is large enough to
warrant its own definition.

Separate the struct dax_dev_range definition and document it prior to
adding information for DC.

Suggested-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Link: https://patch.msgid.link/20241107-dcd-type2-upstream-v7-3-56a84e66bc36@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>