gpio-rcar: Reference core gpio documentation in the DT bindings

Replaced the detailed gpio-ranges documentation with a reference to the
code gpio DT bindings, and mention the gpio flags symbolic names.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>