drm/xe: Expose PCIe link downgrade attributes

drm/xe: Expose PCIe link downgrade attributes

Expose sysfs attributes for PCIe link downgrade capability and status.

v2: Move from debugfs to sysfs (Lucas, Rodrigo, Badal)
    Rework macros and their naming (Rodrigo)
v3: Use sysfs_create_files() (Riana)
    Fix checkpatch warning (Riana)
v4: s/downspeed/downgrade (Lucas, Rodrigo, Riana)
v5: Use PCIe Gen agnostic naming (Rodrigo)
v6: s/pcie_gen/auto_link (Lucas)

Signed-off-by: Raag Jadav <raag.jadav@intel.com>
Reviewed-by: Riana Tauro <riana.tauro@intel.com>
Link: https://lore.kernel.org/r/20250506054835.3395220-3-raag.jadav@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>