arm64: dts: add Protonic PRT8ML board

arm64: dts: add Protonic PRT8ML board

Add devicetree for the Protonic PRT8ML.

The board is similar to the Protonic PRT8MM but i.MX8MP based.

Some features have been removed as the drivers haven't been mainlined
yet or other issues where encountered:
 - Stepper motors to be controlled using motion control subsystem
 - MIPI/DSI to eDP USB alt-mode
 - Onboard T1 ethernet (10BASE-T1L+PoDL, 100BASE-T1+PoDL, 1000BASE-T1)

Signed-off-by: David Jander <david@protonic.nl>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Jonas Rebmann <jre@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>