drm/i915/bxt: add description about the BXT PHYs

Extend the VLV/CHV DPIO (PHY) documentation with the BXT specifics.

v2:
- add more detail about the mapping between ports and transcoders (ville)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>