drm/i915/skl: Added new macros

For SKL, register definition for RPNSWREQ (A008), RPSTAT1(A01C)
have changed slightly. Also on SKL, frequency is specified in
units of 16.66 MHZ, compared to 50 MHZ for most of the earlier
platforms and the time values are expressed in units of 1.33 us,
compared to 1.28 us for earlier platforms.
Added new macros for the aforementioned changes.

v2: Renamed the GT_FREQ_FROM_PERIOD macro to GT_INTERVAL_FROM_US (Damien)

v3: Removed the implicit use of dev_priv in GT_INTERVAL_FROM_US macro (Chris)

Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>