ARM: dts: imx6: add support for the Ka-Ro electronics 'MB7' baseboard

This baseboard can be used with all TX6 SoMs, but only a certain set
of combinations can be ordered by default. Add support for these
combinations in mainline, so that users can easily adopt their own
combination of SoM and baseboard themselves.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>