drm/i915/ehl: Introduce Mule Creek Canyon PCH

Although EHL introduces a new PCH, the South Display part of the PCH
that we care about is nearly identical to ICP, just with some pins
remapped.  Most notably, Port C is mapped to the pins that ICP uses for
TC Port 1.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190615004210.16656-1-matthew.d.roper@intel.com