powerpc/celleb_pci: add a const qualifier

This prepares *of_device_id.data becoming const. Without this change
the following warning would occur:

	arch/powerpc/platforms/cell/celleb_pci.c: In function 'celleb_setup_phb':
	arch/powerpc/platforms/cell/celleb_pci.c:485:11: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

arch/powerpc/platforms/cell/celleb_pci.c

@@ -472,7 +472,7 @@ int __init celleb_setup_phb(struct pci_controller *phb)
 {
 	struct device_node *dev = phb->dn;
 	const struct of_device_id *match;
-	struct celleb_phb_spec *phb_spec;
+	const struct celleb_phb_spec *phb_spec;
 	int rc;
 
 	match = of_match_node(celleb_phb_match, dev);