bus: uniphier-system-bus: add UniPhier System Bus driver

Since commit 1517126fdac2 ("ARM: uniphier: select DM_ETH"), DM-based
drivers/net/smc911x.c is compiled, but it is never probed because the
parent node lacks the DM-based driver.

I need a skeleton driver to populate child devices (but the next commit
will move more hardware settings to the this driver).

I put this to drivers/bus/uniphier-system-bus.c because this is the
same path as the driver in Linux kernel.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/drivers/Kconfig b/drivers/Kconfig
index e34a227..7a839fa 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -10,6 +10,8 @@
 
 source "drivers/axi/Kconfig"
 
+source "drivers/bus/Kconfig"
+
 source "drivers/block/Kconfig"
 
 source "drivers/bootcount/Kconfig"