BCE_REG_MBOX_OUT
bus_write_4(mb->reg, BCE_REG_MBOX_OUT, (uint32_t)msg);
bus_write_4(mb->reg, BCE_REG_MBOX_OUT + 4, (uint32_t)(msg >> 32));
bus_write_4(mb->reg, BCE_REG_MBOX_OUT + 8, 0);
bus_write_4(mb->reg, BCE_REG_MBOX_OUT + 12, 0);