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/bus/Kconfig b/drivers/bus/Kconfig
new file mode 100644
index 0000000..07a33c6
--- /dev/null
+++ b/drivers/bus/Kconfig
@@ -0,0 +1,16 @@
+# SPDX-License-Identifier: GPL-2.0
+#
+# Bus Devices
+#
+
+menu "Bus devices"
+
+config UNIPHIER_SYSTEM_BUS
+	bool "UniPhier System Bus driver"
+	depends on ARCH_UNIPHIER
+	default y
+	help
+	  Support for UniPhier System Bus, a simple external bus.  This is
+	  needed to use on-board devices connected to UniPhier SoCs.
+
+endmenu