mtd: rawnand: mtk: use of_device_get_match_data()

The usage of of_device_get_match_data() reduce the code size a bit.

Also, the only way to call .probe() is to match an entry in
.of_match_table[], so of_device_id cannot be NULL.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Reviewed-by: Xiaolei Li <xiaolei.li@mediatek.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>