remoteproc: imx_rproc: correct firmware reload

ENABLE_M4 should be set to 1 when loading code to TCM, otherwise
you will not able to replace the firmware after you stop m4.

Besides ENABLE_M4, we still need set SW_M4C_RST, because this bit
will be automatically set with SW_M4C_NON_SCLR_RST set.

Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20210602064206.27004-1-peng.fan@oss.nxp.com