drm/i915/icl: add definitions for the ICL PLL registers

There's a lot of code for the PLL enabling, so let's first only
introduce the register definitions in order to make patch reviewing a
little easier.

v2: Coding style (Jani).
v3: Preparation for upstreaming.
v4: Fix MG_CLKTOP2_CORECLKCTL1 address and random typos (James).

Cc: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: James Ausmus <james.ausmus@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180328215803.13835-3-paulo.r.zanoni@intel.com