UBI/cfi-mtd: Fix mtd name for multiple chips

On platforms with multiple NOR chips, currently only the first one
can be selected using the "ubi part" command. This patch fixes this
problem by using different names for the NOR "mtd devices".

It also changes the name of the NOR MTD device from "cfi-mtd" to
"norX" (X indexing the device numer) to better match the mtdparts
defaults.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Andreas Huber <andreas.huber@keymile.com>