arm64: dts: imx8qxp: support scu mailbox channel

With mailbox driver support i.MX8 SCU MU channel, we could
use it to avoid trigger interrupts for each TR/RR registers
in one MU, instead, only one RX interrupt for a recv and
one TX interrupt for a send.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>