boards: nxp: frdm_mcxe31b: add zephyr,shell-uart node

Add missing zephyr,shell-uart device tree node to enable
shell functionality.

Signed-off-by: Lucien Zhao <lucien.zhao@nxp.com>

Author: lucien-nxp

boards/nxp/frdm_mcxe31b/frdm_mcxe31b.dts

@@ -30,6 +30,7 @@
 		zephyr,flash = &program_flash;
 		zephyr,flash-controller = &flash;
 		zephyr,console = &lpuart_5;
+		zephyr,shell-uart = &lpuart_5;
 	};
 
 	leds {