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>