dt-bindings: mxsfb: Add compatible for i.MX8MN

NXP's i.MX8MN has an LCDIF as well.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Link: https://lore.kernel.org/r/20210620225028.189637-1-marex@denx.de


Signed-off-by: Rob Herring <robh@kernel.org>