mailbox: imx: extend irq to an array

To i.MX93 S401 MU, there are two interrupts: rx full and tx empty.
So extend irq to an array to prepare i.MX93 S401 MU support.

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