arm64: dts: renesas: ulcb: add default bootargs

It can't boot without bootargs settings on Uboot on ulcb board.
This patch adds missing default bootargs.
ulcb BSP can overwrite it by own UBoot settings.
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm64/boot/dts/renesas/ulcb.dtsi

@@ -18,6 +18,7 @@ aliases {
 	};
 
 	chosen {
+		bootargs = "ignore_loglevel rw root=/dev/nfs ip=dhcp";
 		stdout-path = "serial0:115200n8";
 	};