bcma: add support for population subnodes also when build as module

of_default_bus_match_table was not exported earlier, so it could only
be accessed by code compiled into the kernel. A new function
of_platform_default_populate() was added which uses
of_default_bus_match_table and this function is also exported. This way
it is possible to create a bus with the content of
of_default_bus_match_table and we can remove the hacks from bcma.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>