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>

arch/arm/mach-mv78xx0/Kconfig

 menuconfig ARCH_MV78XX0
-	bool "Marvell MV78xx0" if ARCH_MULTI_V5
+	bool "Marvell MV78xx0"
+	depends on ARCH_MULTI_V5
 	select ARCH_REQUIRE_GPIOLIB
 	select CPU_FEROCEON
 	select MVEBU_MBUS