mtd: basic (read only) driver for BCMA serial flash

This registers MTD driver for serial flash platform device. Right now it
supports reading only, writing still has to be implemented.

Artem: minor amendments.

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