sh: disallow kexec virtual entry

Older versions of kexec-tools has a zImage loader that
passes a virtual address as entry point. The elf loader
otoh it passes a physical address as entry point, and
pages are always passed as physical addresses as well.

Only allow physical addresses from now on.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>