powerpc/pci: Use of_pci_range_parser helper in pci_process_bridge_OF_ranges

This patch updates the implementation of pci_process_bridge_OF_ranges to use
the of_pci_range_parser helpers.

Signed-off-by: Andrew Murray <amurray@embedded-bits.co.uk>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>