dt-bindings: remoteproc: imx_rproc: Support i.MX8MN/P MMIO

The MX8M CM7 boot via SMC call is problematic, since not all versions
of ATF support this interface. Document MMIO support used to boot the
CM7 on MX8MN/MP instead and discern MMIO interface using DT compatible
string. Document GPR register syscon phandle which is required by the
MMIO interface too.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230724222418.163220-1-marex@denx.de


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