firmware: imx-scu: Support one TX and one RX

Current imx-scu requires four TX and four RX to communicate with
SCU. This is low efficient and causes lots of mailbox interrupts.

With imx-mailbox driver could support one TX to use all four transmit
registers and one RX to use all four receive registers, imx-scu
could use one TX and one RX.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>