mtd: bcm47xxsflash: register this as normal driver

When platform_driver_probe() is used and no device is registered for
this driver -ENODEV is returned and and error message is shown. Not all
BCM47xx SoC have a serial flash chip controller and chip and for them
an error message was shown.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>