of: unittest: 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.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Rob Herring <robh@kernel.org>