This patch adds missing bootargs for R-Car S4 Spider board.

One note is that current Spider board doesn't have Ethernet
support yet, but this patch adds standard settings for it, too.
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87leqo58ox.wl-kuninori.morimoto.gx@renesas.comSigned-off-by: Geert Uytterhoeven <geert+renesas@glider.be>