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/arch/arm/mach-uniphier/Kconfig b/arch/arm/mach-uniphier/Kconfig
index e125f50..3a8eee7 100644
--- a/arch/arm/mach-uniphier/Kconfig
+++ b/arch/arm/mach-uniphier/Kconfig
@@ -83,6 +83,7 @@
 
 config MICRO_SUPPORT_CARD
 	bool "Use Micro Support Card"
+	depends on UNIPHIER_SYSTEM_BUS
 	help
 	  This option provides support for the expansion board, available
 	  on some UniPhier reference boards.