of/platform: add function to populate default bus

When a default bus like the simple-bus should be used someone had to
call of_platform_populate() with the default match table. This match
table was not exported, so it is impossible for code build as a module
to use this. Instead of exporting of_default_bus_match_table, add a new
function which uses this default match table and populates the bus.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Rob Herring <robh@kernel.org>