mtd: bcm63xxpart: make sure CFE and NVRAM partitions are at least 64KiB

The CFE boot loader on BCM63XX platforms assumes itself and the NVRAM
partition to be 64 KiB (or erase block sized, if larger).
Ensure this assumption is also met when creating the partitions to
prevent accidential erasure of CFE or NVRAM.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>