mtd: bcm47xxsflash: store info about flash type

It's going to be needed for erase and write operations, they differ
between Atmel and ST flashes.

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>