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>