ppc64: make sure to align stack pointer to 16 bytes at boot

yaboot is scrogged and calls us with an invalid stack alignment,
it seems.

Thanks to David Woodhouse to pointing me to the problem.
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

arch/powerpc/kernel/head_64.S

@@ -1537,6 +1537,9 @@ _STATIC(__boot_from_prom)
 	mr	r28,r6
 	mr	r27,r7
 
+	/* Align the stack to 16-byte boundary for broken yaboot */
+	rldicr	r1,r1,0,59
+
 	/* Make sure we are running in 64 bits mode */
 	bl	.enable_64b_mode