drm: rcar-du: Add support for external pixel clock

The DU uses the module functional clock as the default pixel clock, but
supports using an externally supplied pixel clock instead. Support this
by adding the external pixel clock to the DT bindings, and selecting the
clock automatically at runtime based on the requested mode pixel
frequency.

The input clock pins to DU channels routing is configurable, but
currently hardcoded to connect input clock i to channel i.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>