MTD: Change cfi-mtd to accept non-uniform sector sizes

With this patch non-uniform NOR FLASH chips (chips with multiple erase
regions) can be exported via the cfi-mtd layer and therefor used by UBI.
We select the largest sector size as erasesize. The cfi driver will make
sure that the smaller sectors are handled correctly.

Signed-off-by: Stefan Roese <sr@denx.de>