remoteproc: imx_rproc: make clk optional

To i.MX7ULP, M4 is the master to control everything, no need to provide
clk from Linux side. So make clk optional when method is IMX_RPROC_NONE.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/1620274123-1461-7-git-send-email-peng.fan@oss.nxp.com


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>