mtd: bcm63xxpart: make fixed part length calculation more generic

The CFE does not use 4K sectors even if the flash supports it, so
for the fixed partitions like CFE itself or NVRAM the erase block
size is always 64k or bigger.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>