MIPS: make PCI_DMA_BUS_IS_PHYS=1 constant

No one of supported MIPS machines has an IOMMU unit, so we can safely define
PCI_DMA_BUS_IS_PHYS = 1. Also remove iommu flag from the pci controller
structure, since it is useless.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Cc: Linux MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/7604/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>