arm64: dts: imx8qxp: enable scu general irq channel

On i.MX8QXP, SCU uses MU1 general interrupt channel #3 to notify
user for IRQs of RTC alarm, thermal alarm and WDOG etc., mailbox
RX doorbell mode is used for this function, this patch adds
support for it.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>