This commit adds support for NXP board LPCXpresso55S69 for mbox.
- samples/drivers/mbox/ - mbox signaling mode
- samples/drivers/mbox_data/ - mbox data transfer mode
- tests/drivers/mbox/mbox_data/ - mbox test to verify functionality.
Signed-off-by: Tomas Galbicka <tomas.galbicka@nxp.com>