dt-bindings: mailbox: fsl,mu: add new tx doorbell channel

Add new tx doorbell channel for i.MX95 SCMI mailbox usage.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>