remoteproc: ti_k3_m4_remoteproc: Fix build warnings around k3_m4_rproc_stop()

Fix below warning:

 drivers/remoteproc/ti_k3_m4_remoteproc.c: In function 'k3_m4_rproc_stop':
   drivers/remoteproc/ti_k3_m4_remoteproc.c:379:46: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
     379 |         ret = mbox_send_message(kproc->mbox, (void *)msg);
         |                                              ^

Fixes: 99cff963

 ("remoteproc: k3-m4: extend stop to send shutdown message to m4 core")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>