drm/i915/skl: Documentation for CSR firmware

Added docbook info regarding context save and restore (CSR)
firmware support added from gen9 onwards to drive newly added
DMC (Display microcontroller) in display engine.

v1: Initial version as RFC.

v2: Used "DOC:" tag for csr description based on review comment from Daniel.

Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: A.Sunil Kamath <sunil.kamath@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>