bus: uniphier-system-bus: revive tristate prompt

At first, commit 4b7f48d3 ("bus: uniphier-system-bus: add UniPhier
System Bus driver") introduced this driver as a tristate one.

Then, commit 326ea45a ("bus: uniphier: allow only built-in
driver") temporarily made it boolean in order to fix a link error
in case it is compiled as a module.

The root cause was fixed by commit b80443c2 ("of/platform: export
of_default_bus_match_table").

Now this driver can really be a module.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Olof Johansson <olof@lixom.net>