dt-bindings: display: renesas: lvds: Add EXTAL and DU_DOTCLKIN clocks

On the D3 and E3 SoCs, the LVDS encoder can derive its internal pixel
clock from an externally supplied clock, either through the EXTAL pin or
through one of the DU_DOTCLKINx pins. Add corresponding clocks to the DT
bindings.

To retain backward compatibility with DT that don't specify the
clock-names property, the functional clock must always be specified
first, and the clock-names property is optional when only the functional
clock is specified.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>