remoteproc: imx_rproc: Switch iMX8MN/MP from SMCCC to MMIO

The MX8M CM7 boot via SMC call is problematic, since not all versions
of ATF support this interface. Extend the MMIO support so it can boot
the CM7 on MX8MN/MP instead and discern the two alternatives using DT
compatible strings.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20230724222418.163220-2-marex@denx.de


Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>