arm: fsl: imx8mn_bsh_smm_s2: Migrate to OF_UPSTREAM

Migrate imx8mn_bsh_smm_s2 and imx8mn_bsh_smm_s2pro boards to OF_UPSTREAM.

Signed-off-by: Patrick Barsanti <patrick.barsanti@amarulasolutions.com>
Tested-by: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>