mtd: bcm47xxpart: add support for other erase sizes

To make the partitions writable they should aligned to erase sizes of
the flash. If the erase size is small use 0x10000.

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