remoteproc: imx_dsp_rproc: Fix multiple start/stop operations

remoteproc: imx_dsp_rproc: Fix multiple start/stop operations

After commit 67a7bc7f0358 ("remoteproc: Use of reserved_mem_region_*
functions for "memory-region"") following commands with
imx-dsp-rproc started to fail:

$ echo zephyr.elf > /sys/class/remoteproc/remoteproc0/firmware
$ echo start > /sys/class/remoteproc/remoteproc0/state
$ echo stop > /sys/class/remoteproc/remoteproc0/state
$ echo start > /sys/class/remoteproc/remoteproc0/state #! This fails
-sh: echo: write error: Device or resource busy

This happens because aforementioned commit replaced devm_ioremap_wc with
devm_ioremap_resource_wc which will "reserve" the memory region with the
first start and then will fail at the second start if the memory
region is already reserved.

Even partially reverting the faulty commit won't fix the
underlying issue because we map the address in prepare() but we never
unmap it at unprepare(), so we will keep leaking memory regions.

So, lets use alloc() and release() callbacks for memory carveout
handling. This will nicely map() the memory region at prepare() time
and unmap() it at unprepare().

Fixes: 67a7bc7f0358 ("remoteproc: Use of_reserved_mem_region_* functions for "memory-region"")
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://lore.kernel.org/r/20251210154906.99210-1-daniel.baluta@nxp.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>