powerpc/powernv: Don't clobber r9 in relative_toc()

With OPAL, r8 and r9 will be used to pass the OPAL base and entry
for debugging purposes (those informations are also in the
device-tree). We don't want to clobber those registers that
early.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/head_64.S

@@ -674,9 +674,9 @@ _GLOBAL(enable_64b_mode)
 _GLOBAL(relative_toc)
 	mflr	r0
 	bcl	20,31,$+4
-0:	mflr	r9
-	ld	r2,(p_toc - 0b)(r9)
-	add	r2,r2,r9
+0:	mflr	r11
+	ld	r2,(p_toc - 0b)(r11)
+	add	r2,r2,r11
 	mtlr	r0
 	blr