mtd: bcm47xxsflash: implement ChipCommon R/W ops

They are needed for erasing/writing. Use a magic pointers and small
functions to prepare code for adding other buses support in the future
(like SSB).

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>