drm/i915/bxt: Define BXT power domains

Add BXT power domains

v2: Use DOMAIN_PLLS instead of a new CDCLK one, whitespace fixes
    (Damien)
v3: add VGA, TRANSCODER_A power domains (imre)

Signed-off-by: Satheeshakrishna M <satheeshakrishna.m@intel.com> (v1)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
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>