tools: mtk_image: add support for MT7621 NAND images

The BootROM of MT7621 requires a image header for SPL to record its size
and load address when booting from NAND.

To create such an image, one can use the following command line:
mkimage -T mtk_image -a 0x80200000 -e 0x80200000 -n "mt7621=1"
-d u-boot-spl-ddr.bin u-boot-spl-ddr.img

Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>