cxl: Add Get Supported Features command for kernel usage

CXL spec r3.2 8.2.9.6.1 Get Supported Features (Opcode 0500h)
The command retrieve the list of supported device-specific features
(identified by UUID) and general information about each Feature.

The driver will retrieve the Feature entries in order to make checks and
provide information for the Get Feature and Set Feature command. One of
the main piece of information retrieved are the effects a Set Feature
command would have for a particular feature. The retrieved Feature
entries are stored in the cxl_mailbox context.

The setup of Features is initiated via devm_cxl_setup_features() during the
pci probe function before the cxl_memdev is enumerated.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Li Ming <ming.li@zohomail.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Tested-by: Shiju Jose <shiju.jose@huawei.com>
Link: https://patch.msgid.link/20250220194438.2281088-3-dave.jiang@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

drivers/cxl/cxlmem.h

@@ -392,6 +392,7 @@ struct cxl_dpa_perf {
  * @serial: PCIe Device Serial Number
  * @type: Generic Memory Class device or Vendor Specific Memory device
  * @cxl_mbox: CXL mailbox context
+ * @cxlfs: CXL features context
  */
 struct cxl_dev_state {
 	struct device *dev;
@@ -407,6 +408,9 @@ struct cxl_dev_state {
 	u64 serial;
 	enum cxl_devtype type;
 	struct cxl_mailbox cxl_mbox;
+#ifdef CONFIG_CXL_FEATURES
+	struct cxl_features_state *cxlfs;
+#endif
 };
 
 static inline struct cxl_dev_state *mbox_to_cxlds(struct cxl_mailbox *cxl_mbox)