mtd, ubi, ubifs: license file adaptions

replace the GPL license file header from Linux with the
SPDX-License-Identifier.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Sergey Lapin <slapin@ossfans.org>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Wolfgang Denk <wd@denx.de>