mtd: bcm47xxpart: look for NVRAM at the end of device

NVRAM is always placed at the end of device and it does not have to
start at the beginning of a block, so check few possible offsets.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>