mtd: bcm47xxpart: find NVRAM partitions in middle blocks

Old devices used to have NVRAM at the very end of flash and they could
be unaligned (starting at some offset in a block).
In new devices NVRAM can be located quite randomly, however it seems to
always start at the beginning of a block. For example Netgear R6250 has
NVRAM located right after the bootloader, before the kernel partition.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>