microblaze: Add definition for PCI_DMA_BUS_IS_PHYS in pci.h

Add definition for PCI_DMA_BUS_IS_PHYS in pci.h, which enables support for the
SCSI library. This is to support USB Mass Storage devices, such as thumb
drives.

Signed-off-by: Julie Zhu <julie.zhu@xilinx.com>
Signed-off-by: Michal Simek <monstr@monstr.eu>