Several boards defining mtd partitions also defined NAND_BLOCK_SIZE as
SZ_128K. Move the define to common-board-devices.h

This removes multiple defines of NAND_BLOCK_SIZE.
Signed-off-by: NIgor Grinberg <grinberg@compulab.co.il>
[tony@atomide.com: updated comments]
Signed-off-by: NTony Lindgren <tony@atomide.com>