mtd: bcm47xxpart: improve probing of nvram partition

The nvram in the nvram partition does not start at the beginning of the
partition on every device. Sometimes they are stating in the middle of
a partition or the first 0x1000 bytes are free.

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