ARM: rmobile: Add R8A77965 Salvator-XS board support

Add R8A77965 M3N Salvator-XS development kit support. This kit is
similar to the other Salvator-X(S) ones, except is has M3N SoC on
it.

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