cxl: Move hpa_to_spa callback to a new root decoder ops structure

cxl: Move hpa_to_spa callback to a new root decoder ops structure

The root decoder's HPA to SPA translation logic was implemented using
a single function pointer. In preparation for additional per-decoder
callbacks, convert this into a struct cxl_rd_ops and move the
hpa_to_spa pointer into it.

To avoid maintaining a static ops instance populated with mostly NULL
pointers, allocate the ops structure dynamically only when a platform
requires overrides (e.g. XOR interleave decoding).

The setup can be extended as additional callbacks are added.

Co-developed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Link: https://patch.msgid.link/818530c82c351a9c0d3a204f593068dd2126a5a9.1754290144.git.alison.schofield@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>