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>

drivers/bus/Kconfig

@@ -132,7 +132,7 @@ config SUNXI_RSB
 	  and AC100/AC200 ICs.
 
 config UNIPHIER_SYSTEM_BUS
-	tristate "UniPhier System Bus driver"
+	bool "UniPhier System Bus driver"
 	depends on ARCH_UNIPHIER && OF
 	default y
 	help