ARM: dts: uniphier: compile only DT files that make sense

All the UniPhier DT files are compiled if CONFIG_ARCH_UNIPHIER
is enabled, but not all of them actually work.  For example, when
U-Boot is compiled for ARM 32 bit, 64 bit DT files are also built,
and vice versa.  Compile only the combination that makes sense.

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