cfi_flash: Export flash_sector_size() function.

Export flash_sector_size() function from drivers/mtd/cfi_flash.c,
so that it can be used in the upcoming cfi-mtd driver.

Signed-off-by: Piotr Ziecik <kosmo@semihalf.com>
Signed-off-by: Stefan Roese <sr@denx.de>