bcma: populate bus DT subnodes as platform_device-s

Our bus should allow defining children nodes as we may want to specify
devices attached to the bus. This is required e.g. to specify NAND or
ChipCommon cores and use bus's address and IRQ mappings.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>