tools: mtk_image: add support for booting ARM64 images

mkimage is only able to package aarch32 binaries. Add support for
AArch64 images.

One can create a ARM64 image using the following command line:
mkimage -T mtk_image -a 0x201000 -e 0x201000 -n "media=emmc;arm64=1"
-d bl2.bin bl2.img

Signed-off-by: Fabien Parent <fparent@baylibre.com>