drivers: mbox: Add nxp mbox multi-channel support

Current implementation of NXP mbox driver mbox_nxp_imx_mu
is using only one channel 0.
This commit adds support for multiple mbox channels as is
indented by mbox drivers.

Change done in .send api signaling mode leveraging provided
channel id to select correct General Purpose Interrupt.
Another change done in IRQHandler to check and handle all
channels.

Signed-off-by: Tomas Galbicka <tomas.galbicka@nxp.com>