drivers: mbox: initialize before ipc

Change the default mbox initialization priority so that it initializes
before ipc, as some ipc drivers depends on it.

Found with:

$ west build -p -b nrf5340dk_nrf5340_cpuapp \
		samples/subsys/ipc/ipc_service/icmsg_me \
		-DCONFIG_CHECK_INIT_PRIORITIES=y
...
ERROR: /ipc/ipc1 POST_KERNEL 46 <
	/soc/peripheral@50000000/mbox@2a000 POST_KERNEL 50
ERROR: /ipc/ipc0 POST_KERNEL 46 <
	/soc/peripheral@50000000/mbox@2a000 POST_KERNEL 50

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>