drm/i915: add new "soc" sub-directory and move PCH and DRAM code there

Add a new sub-directory for things that aren't specifically about the
GPU and don't really belong in the i915 driver top level, but also don't
belong under any of the existing sub-directories either.

Name it "soc", and move the PCH and DRAM code there.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221208142347.602726-1-jani.nikula@intel.com