ARM: rmobile: salvator-x: Add R8A7796 support

Add minor ifdeffery and default board config for the Salvator-XS board
with R8A7796 M3 SoC.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>