arm64: dts: imx8mm-kontron: Add support for display bridges on BL i.MX8MM

The Kontron Electronics BL i.MX8MM has oboard disply bridges for
DSI->HDMI and DSI->LVDS conversion. The DSI interface is muxed by
a GPIO-controlled switch to one of these two bridges.

By default the HDMI bridge is enabled. The LVDS bridge can be
selected by loading an additional (panel-specific) overlay.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>