mtd: rawnand: mtk: Re-license MTK NAND driver as Dual MIT/GPL

It is wanted to use MTK NAND driver with GPL-2.0 or MIT license.
But now it is only licensed as GPL-2.0, so re-license it as dual
MIT/GPL.

Signed-off-by: Xiaolei Li <xiaolei.li@mediatek.com>
Acked-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>
Acked-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Wen Yang <yellowriver2010@hotmail.com>
Acked-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: RogerCC Lin <rogercc.lin@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>