of/platform: export of_default_bus_match_table

Currently, drivers/bus/uniphier-system-bus.c is kept from being a
module due to the unresolved reference to of_default_bus_match_table.

Refer to commit 326ea45a ("bus: uniphier: allow only built-in
driver").

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rob Herring <robh@kernel.org>