drm/i915/skl: Structure/enum definitions for SKL clocks

Adding structure/enum for SKL clocking implementation.

v2: Addressed Damien's comment
	- Removed internal structure from this header file

v3: Stove this into the generic intel_dpll_id enum and give them the established
DPLL_ID_ prefixes. (Daniel)

v4: - We'll only try to share DPLL1/2/3, leaving DPLL0 to eDP
    - Use SKL in the skylake shared DPLL names
    - Re-add the skl_dpll enum
    (Damien)

v5: Remove SKL_DPLL_NONE (Daniel)

v6: Modified as per review comments from Paulo

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Satheeshakrishna M <satheeshakrishna.m@intel.com> (v2)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com> (v4,v5)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v3)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>