boards: beaglebone_ai64: Add IPC support

Add IPC memory and mailbox chosen properties. The mailbox channels
used are as expected by Linux.

Signed-off-by: Andrew Davis <afd@ti.com>

Author: glneo

boards/beagle/beaglebone_ai64/beaglebone_ai64_j721e_main_r5f0_0.dts

@@ -20,6 +20,8 @@
 		zephyr,sram = &atcm;
 		zephyr,console = &uart2;
 		zephyr,shell-uart = &uart2;
+		zephyr,ipc = &ipc0;
+		zephyr,ipc_shm = &ddr0;
 	};
 
 	cpus {
@@ -44,6 +46,12 @@
 		reg = <0xa2200000 DT_SIZE_M(14)>;
 		zephyr,memory-region = "DRAM";
 	};
+
+	ipc0: ipc {
+		compatible = "zephyr,mbox-ipm";
+		mboxes = <&mbox1 0>, <&mbox1 1>;
+		mbox-names = "tx", "rx";
+	};
 };
 
 &i2c6 {