Add the image type handler to the ARM64 boot code. The only difference
in the boot handling is that we need to look at the FIT loaded OS image
header to get the image load and text offsets, as the os_header is the
FIT header, not the kernel header.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>