Portenta H7 add chosen uart0mcumgr define

Partial fix for USB Serial not working

Author: KurtE

loader/boards/arduino_portenta_h7_stm32h747xx_m7.overlay

@@ -34,6 +34,7 @@
 		zephyr,console = &usart6;
 		zephyr,shell-uart = &usart6;
 		zephyr,cdc-acm-uart0 = &usart6;
+		zephyr,uart-mcumgr = &usart6;
 	};
 };