arm64: dts: imx8mp-hummingboard-iiot: add dt overlays for muxable ports

arm64: dts: imx8mp-hummingboard-iiot: add dt overlays for muxable ports

The SolidRun i.MX8MP HummingBoard IIoT has a variety of connectors, and
configurable ports:

- 2x RS232
- 2x RS485
- DSI Panel Connector
- LVDS Panel Connector

RS232 and RS485 each share a single UART from the SoC via a mux, which
are configured from the base dts for RS232 by default.

Provide addons for:

1. Reconfigure UART A from RS232 to RS485.
2. Reconfigure UART B from RS232 to RS485.
3. Configure DSI Connector for Winstar WJ70N3TYJHMNG0 Panel.
4. Configure LVDS Connector for Winstar WF70A8SYJHLNGA Panel.

A variation of the base dtb is generated for each addon to ensure that
make dtbs_check covers the resulting dtb, and applying overlay is
tested during build.

It is however expected that bootloader should apply any combination of
addons based on runtime configuration.

Signed-off-by: Josua Mayer <josua@solid-run.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>