bus: uniphier-system-bus: use of_platform_default_populate() to populate default bus

Use helper of_platform_default_populate() in linux/of_platform
when possible, instead of calling of_platform_populate() with
the default match table.

Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Rob Herring <robh@kernel.org>