ARM: dts: imx7: add MIPI-DSI support

This adds the device tree support for the MIPI-DSI block. The block can
be used as encoder for the parallel signals coming from the lcdif block.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Roland Hieber <rhi@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>