powerpc: Fix compiler warning for the relocatable kernel

Fixes this warning:
 arch/powerpc/kernel/setup_64.c:447:5: warning: "kernstart_addr" is not defined

which arises because PHYSICAL_START is no longer a constant when
CONFIG_RELOCATABLE=y.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>