bcma: make OF code more generic (not platform_device specific)

OF allows not only specifying platform devices but also describing
devices on standard buses like PCI or USB. This change will allow
reading info from DT for bcma buses hosted on PCI cards.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>