samples: openthread: coprocessor: Add chosen zephyr,ot-uart

As per upstream commit f460848, the
OpenThread UART instance is now configured with DT instead of Kconfig.

Signed-off-by: Robert Lubos <[email protected]>

Author: rlubos

samples/openthread/coprocessor/boards/nrf21540dk_nrf52840.overlay

@@ -25,5 +25,6 @@
 	*/
 	chosen {
 		zephyr,entropy = &rng;
+		zephyr,ot-uart = &uart0;
 	};
 };

samples/openthread/coprocessor/boards/nrf52833dk_nrf52833.overlay

@@ -13,3 +13,9 @@
 	current-speed = <1000000>;
 	status = "okay";
 };
+
+/ {
+	chosen {
+		zephyr,ot-uart = &uart0;
+	};
+};

samples/openthread/coprocessor/boards/nrf52840dk_nrf52840.overlay

@@ -23,5 +23,6 @@
 	*/
 	chosen {
 		zephyr,entropy = &rng;
+		zephyr,ot-uart = &uart0;
 	};
 };