remoteproc: imx_dsp_rproc: Add remoteproc driver for DSP on i.MX

Provide a basic driver to control DSP processor found on NXP i.MX8QM,
i.MX8QXP, i.MX8MP and i.MX8ULP.

Currently it is able to resolve addresses between DSP and main CPU,
start and stop the processor, suspend and resume.

The communication between DSP and main CPU is based on mailbox, there
are three mailbox channels (tx, rx, rxdb).

This driver was tested on NXP i.MX8QM, i.MX8QXP, i.MX8MP and i.MX8ULP.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1633944015-789-4-git-send-email-shengjiu.wang@nxp.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>