mtd: nand/onenand: Register mtd device upon device scanning

With this patch the NAND and OneNAND devices are registered in the MTD
subsystem and can then be referenced by the mtdcore code (e.g.
get_mtd_device_nm()). This is needed for the new "ubi part" command
syntax without the flash type parameter (nor|nand|onenand).

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