cfi-mtd: Add cfi-mtd driver.

Add cfi-mtd driver, which exports CFI flash to MTD layer.
This allows CFI flash devices to be used from MTD layer.

Building of the new driver is controlled by CONFIG_FLASH_CFI_MTD
option. Initialization is done by calling cfi_mtd_init() from
flash_init().

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