arm64: dts: imx: add Protonic PRT8MM board

The Protonic PRT8MM is a low-cost agricultural Virtual Terminal. This
commit adds most of the board functionality sans the display output,
as the i.MX8MM display support isn't ready yet.

Signed-off-by: David Jander <david@protonic.nl>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>