ARM: kexec: remove 512MB restriction on kexec crashdump

The real limit is the top of the visible physical address space with
the MMU turned off.  Hence, we need to limit the crash kernel allocation
running-view physical address of the top of the boot-view physical
address space.

Reviewed-by: Pratyush Anand <panand@redhat.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>