ARM: uniphier: prepare directory structure for ARMv8 SoC support

Before adding ARMv8 support, this commit refactors the directory
structure.  Move ARMv7 specific files to arch/arm/mach-uniphier/arm32
to avoid a mess by mixture of ARMv7 and ARMv8 code.  Also move the
"select CPU_V7" to the lower-level menu because we will have to
select ARM64 instead of CPU_V7 for ARMv8 SoCs.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>