powerpc/kexec: Adds correct calling convention for kexec purgatory

Call kexec purgatory code correctly. We were getting lucky before.
If you examine the powerpc 32bit kexec "purgatory" code you will
see it expects the following:

>From kexec-tools: purgatory/arch/ppc/v2wrap_32.S
-> calling convention:
->   r3 = physical number of this cpu (all cpus)
->   r4 = address of this chunk (master only)

As such, we need to set r3 to the current core, r4 happens to be
unused by purgatory at the moment but we go ahead and set it
here as well
Signed-off-by: NMatthew McClintock <msm@freescale.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/misc_32.S

@@ -810,6 +810,9 @@ relocate_new_kernel:
 	isync
 	sync
 
+	mfspr	r3, SPRN_PIR /* current core we are running on */
+	mr	r4, r5 /* load physical address of chunk called */
+
 	/* jump to the entry point, usually the setup routine */
 	mtlr	r5
 	blrl