drm/i915: Introduce AUX_CH_USBCn

Just like with the DDIs tgl+ renamed the AUX CHs to reflect
the type of the DDI. Let's add the aliasing enum values for
the type-C AUX CHs.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201028213323.5423-5-ville.syrjala@linux.intel.com