dt-bindings: mailbox: imx-mu: add generic MU channel support

Each MU has four pairs of rx/tx data register with four rx/tx interrupts
which can also be used as a separate channel.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>