Documentation/soc/intel: Add common code design document

Add common code design document support Intel SoCs such as Skylake,
Cannonlake and Apollolake onwards.

Documented items:
*Introduction
*Design Principle
*Common code development and status
*Common code structure
*Benifits

Change-Id: I5ade390cfb41c72f812d5cc4e00e67a5964721de
Signed-off-by: Maulik V Vaghela <maulik.v.vaghela@intel.com>
Reviewed-on: https://review.coreboot.org/27087
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>
2 files changed