mtd: prepare place for BCMA NAND flash driver(s)

BCMA bus can contain NAND flash memory, it's registered in system as
platform device. This adds required hooks and place for controler
specific drivers.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>