drm/i915/cx0: Add MTL+ Thunderbolt PLL hooks

drm/i915/cx0: Add MTL+ Thunderbolt PLL hooks

Add the PLL hooks for the TBT PLL on MTL+. These are simple stubs
similarly to the TBT PLL on earlier platforms, since this PLL is always
on from the display POV - so no PLL enable/disable programming is
required as opposed to the non-TBT PLLs - and the clocks for different
link rates are enabled/disabled at a different level, via the
intel_encoder::enable_clock()/disable_clock() interface.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/20251117104602.2363671-32-mika.kahola@intel.com