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>