ARM: Fix relocation to move past the running code

Otherwise we end up overwriting ourselves partially when relocating
less than size of the running code in head.S.

Without this patch, a system will not boot if the compressed image
load address is slightly less than where the compressed image gets
relocated.

For example, using mkimage to set the load address to something like
zreladdr + uncompressed image size - 0x100 will make the system hang
without this patch.

Signed-off-by: Tony Lindgren <tony@atomide.com>