cxl/pci: Introduce cxl_gpf_get_dvsec()

cxl/pci: Introduce cxl_gpf_get_dvsec()

Add a helper to fetch the port/device GPF dvsecs. This is
currently only used for ports, but a later patch to export
dirty count to users will make use of the device one.

Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Li Ming <ming.li@zohomail.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://patch.msgid.link/20250220220235.276831-2-dave@stgolabs.net
Signed-off-by: Dave Jiang <dave.jiang@intel.com>