Documentation: drm: describing drm properties exposed by various drivers

Started documenting drm properties for drm drivers. This patch provides
information about properties in drm, i915, psb and cdv/gma-500. Information
about other properties can be added on top of these.

v2: Added description of drm properties in armada, exynos, i2c/ch7006, noveau,
omap, qxl, radeon, rcar-du

v3: Removed "Property Object" column since it is implementation related. Property
type column refined.[Ville's review comments]

v4: Removed whitespace warnings and minor nits. [Randy's review comments]

v5: Restructured output for ENUM properties

v6: Review comments on formatting the table. [Laurent's review comments]

v7: Minor restructuring. [Laurent's review comments]

Cc: Rob Landley <rob@landley.net>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Cc: Sagar Kamble <sagar.a.kamble@intel.com>
Cc: "Purushothaman, Vijay A" <vijay.a.purushothaman@intel.com>
Cc: linux-doc@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Sagar Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>