mailbox: imx: support dual interrupts

i.MX93 S401 MU support two interrupts: tx empty and rx full.

 - Introduce a new flag IMX_MU_V2_IRQ for the dual interrupt case
 - Update Copyright

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