remoteproc: imx_dsp_rproc: Simplify IMX_RPROC_MMIO switch case

remoteproc: imx_dsp_rproc: Simplify IMX_RPROC_MMIO switch case

Introduce imx_dsp_rproc_mmio_{start, stop, detect_mode}() helper functions
for i.MX variants using IMX_RPROC_MMIO to manage remote processors.

Allows the removal of the IMX_RPROC_MMIO switch-case blocks from
imx_dsp_rproc_[start,stop,detect_mode](), resulting in cleaner and more
maintainable code.

No functional changes.

Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Iuliana Prodan <iuliana.prodan@nxp.com>
Tested-by: Iuliana Prodan <iuliana.prodan@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20251119-imx-dsp-2025-11-19-v4-8-adafd342d07b@nxp.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>