ARM: uniphier: check DT to determine whether to initialize SBC

ARM: uniphier: check DT to determine whether to initialize SBC

If CONFIG_MICRO_SUPPORT_CARD is enabled, U-Boot initialize SBC
(System Bus Controller), which may not be really necessary.

Check the "socionext,uniphier-system-bus" node in DT run-time.
If and only if it is found and its "status" property is okay,
initialize the SBC block.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>