remoteproc: imx_rproc: Simplify IMX_RPROC_MMIO switch case

remoteproc: imx_rproc: Simplify IMX_RPROC_MMIO switch case

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

This allows the removal of the IMX_RPROC_MMIO switch-case blocks from
imx_rproc_start(), imx_rproc_stop(), and imx_rproc_detect_mode(), resulting
in cleaner and more maintainable code.

No functional changes.

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20250910-imx-rproc-cleanup-v2-3-10386685b8a9@nxp.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>