remoteproc: imx_rproc: add a NXP/Freescale imx_rproc driver

Provide a basic driver to control Cortex M4 co-processor found
on NXP i.MX7D and i.MX6SX.
Currently it is able to resolve addresses between M4 and main CPU,
start and stop the co-processor. Other functionality is not provided
or test.

This driver was tested on NXP i.MX7D and expected to work on
i.MX6SX as well.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>