verdin-imx8mm: implement hardware version detection

And select the correct devicetree accordingly by setting the variant
environment variable.

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>