mtd: bcm47xxsflash: keep a reference to the BCMA

To implement erase and write support we need to "talk" with ChipCommon
BCMA core which serial flash it attached to.

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>