ARM: dts: DRA74x: Add IPC sub-mailbox nodes for all IPUs & DSPs

Add the sub-mailbox nodes that are used to communicate between
MPU and the remote processors IPU1, IPU2, DSP1 and DSP2.

The sub-mailbox nodes utilize the System Mailbox instances 5 and 6.
These sub-mailbox nodes are added to match the hard-coded mailbox
configuration used within the TI IPC 3.x software package. The
Dual-Cortex M4 IPU1 and IPU2 processor sub-systems are assumed to
be running in SMP-mode, and hence only a single sub-mailbox node
is added for each.

All these sub-mailbox nodes are left in disabled state, and should
be enabled (and modified if needed) as per the individual product
configuration in the corresponding board dts files.

Signed-off-by: Suman Anna <[e-mail]>
Signed-off-by: Tony Lindgren <[e-mail]>

arch/arm/boot/dts/dra74x.dtsi

···
 		 <&dss_video2_clk>;
 	clock-names = "fck", "video1_clk", "video2_clk";
 };
+
+&mailbox5 {
+	mbox_ipu1_ipc3x: mbox_ipu1_ipc3x {
+		ti,mbox-tx = <6 2 2>;
+		ti,mbox-rx = <4 2 2>;
+		status = "disabled";
+	};
+	mbox_dsp1_ipc3x: mbox_dsp1_ipc3x {
+		ti,mbox-tx = <5 2 2>;
+		ti,mbox-rx = <1 2 2>;
+		status = "disabled";
+	};
+};
+
+&mailbox6 {
+	mbox_ipu2_ipc3x: mbox_ipu2_ipc3x {
+		ti,mbox-tx = <6 2 2>;
+		ti,mbox-rx = <4 2 2>;
+		status = "disabled";
+	};
+	mbox_dsp2_ipc3x: mbox_dsp2_ipc3x {
+		ti,mbox-tx = <5 2 2>;
+		ti,mbox-rx = <1 2 2>;
+		status = "disabled";
+	};
+};