ARM: rmobile: Unify Gen3 Salvator-X(S) and ULCB defconfigs

The r8a779{5,6,65}_salvator-x and r8a779{5,6,65}_ulcb_defconfig were
building the same target, except for the default DT. The default DT is
however only a detail, as the actual DT to be used to configure U-Boot
is detected automatically based on the CPU ID, hence the default DT is
not meaningful. Unify each three defconfigs per board to reduce the
duplication.

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