MIPS: PCI: make phys_to_dma/dma_to_phys for pci-xtalk-bridge common

All platforms using pci-xtalk-bridge can share common phys_to_dma/
dma_to_phys function. So we move it form ip27 specific file to
pci-xtalk-bridge.c

Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@mips.com>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org