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>

arch/arm64/boot/dts/freescale/imx8qxp.dtsi

@@ -21,6 +21,7 @@
 		mmc1 = &usdhc2;
 		mmc2 = &usdhc3;
 		serial0 = &adma_lpuart0;
+		mu1 = &lsio_mu1;
 	};
 
 	cpus {
@@ -117,7 +118,8 @@
 	scu {
 		compatible = "fsl,imx-scu";
 		mbox-names = "tx0", "tx1", "tx2", "tx3",
-			     "rx0", "rx1", "rx2", "rx3";
+			     "rx0", "rx1", "rx2", "rx3",
+			     "gip3";
 		mboxes = <&lsio_mu1 0 0
 			  &lsio_mu1 0 1
 			  &lsio_mu1 0 2
@@ -125,7 +127,8 @@
 			  &lsio_mu1 1 0
 			  &lsio_mu1 1 1
 			  &lsio_mu1 1 2
-			  &lsio_mu1 1 3>;
+			  &lsio_mu1 1 3
+			  &lsio_mu1 3 3>;
 
 		clk: clock-controller {
 			compatible = "fsl,imx8qxp-clk";