bus: uniphier: allow only built-in driver

Building the newly added uniphier system bus driver as a module
causes a link error, so let's only allow it to be built-in for
now, to fix allmodconfig:

ERROR: "of_default_bus_match_table" [drivers/bus/uniphier-system-bus.ko] undefined!

Signed-off-by: Arnd Bergmann <arnd@arndb.de>