cxl/core: Rename ->decoder_range ->hpa_range

In preparation for growing a ->dpa_range attribute for endpoint
decoders, rename the current ->decoder_range to the more descriptive
->hpa_range.

Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Adam Manzanares <a.manzanares@samsung.com>
Link: https://lore.kernel.org/r/165603872867.551046.2170426227407458814.stgit@dwillia2-xfh
Signed-off-by: Dan Williams <dan.j.williams@intel.com>