cxl/port: Move dport probe operations to a driver event

cxl/port: Move dport probe operations to a driver event

In preparation for adding more register setup to the cxl_port_add_dport()
path (for RAS register mapping), move the dport creation event to a driver
callback. This achieves two goals, it puts driver operations logically
where they belong, in a driver, and it obviates the gymnastics of
DECLARE_TESTABLE() which just makes a mess of grepping for CXL symbols.

In other words, a driver callback is less of an ongoing maintenance burden
than this DECLARE_TESTABLE arrangement that does not scale and diminishes
the grep-ability of the codebase.

cxl_port_add_dport() moves mostly unmodified from drivers/cxl/core/port.c.
The only deliberate change is that it now assumes that the device_lock is
held on entry and the driver is attached (just like cxl_port_probe()).

Reviewed-by: Terry Bowman <terry.bowman@amd.com>
Tested-by: Terry Bowman <terry.bowman@amd.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Link: https://patch.msgid.link/20260131000403.2135324-6-dan.j.williams@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>