ARM: uniphier: rework uniphier_set_fdt_file()

The current table look-up for the DTB file name turned out bothersome
in terms of maintainability; I ended up adding a new entry every time
a new board is supported.

There is a common pattern between the DT compatible string and the
corresponding file name; drop the vendor prefix "socionext," and
prefix it with "uniphier-" and suffix it with ".dtb".

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