dt-bindings: display: tfp410: Add bus-width parameter property

tfp410 can be connect to host processor in 24bit, single-edge (24 lines) or
12bit, dual-edge (12 lines).

Add bus-width to the documentation so it can be used to select between the
two connection scheme.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190401124143.17179-2-peter.ujfalusi@ti.com