bcma: use separated function to initialize bus on SoC

This is required to split SoC bus init into two phases. The later one
(which includes scanning) should be called when kalloc is available.

Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>