dt-bindings: arm: fsl: Add iMX8MN BSH SMM S2 boards

Add bindings for BSH SystemMaster (SMM) S2 board family, which consists
of: iMX8MN SMM S2 and iMX8MN SMM S2 PRO boards.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>