sh: kexec: Add PHYSICAL_START

Add PHYSICAL_START kernel configuration parameter to set the address at
which the kernel should be loaded.

It has been observed on an sh7757lcr that simply modifying MEMORY_START
does not achieve this goal for 32bit sh. This is due to MEMORY_OFFSET in
arch/sh/kernel/vmlinux.lds.S bot being based on MEMORY_START on such
systems.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>