ARM: mv78xx0: use "depends on" instead of "if" after prompt

This platform recently moved to multi-platform, so missed the global
fixup by commit e3246542

 ("ARM: use "depends on" for SoC configs
instead of "if" after prompt").  Fix it now.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>