ARM: dts: imx51: Add M4IF support

As per the i.MX51 Reference Manual the M4IF register region
starts at 0x83fd8000 and has a 4kB address range.

Add support for it.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Sergey Lapin <sergey.lapin@cogentembedded.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>