arm64: dts: uniphier: use cross-arch include instead of symlinks

On UniPhier platform, some DTSI files are shared between arm and arm64.
Recently, inclusion of DT material of different architectures has been
supported by the build system level.  Use #include <arm/...>, which
will work without relying on the exact same hierarchy as the kernel.

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