bus/pci: support IOVA as VA on PowerNV systems

bus/pci: support IOVA as VA on PowerNV systems

All recent POWER systems, Power 8 and 9 specifically, support an IOMMU
(it can't be disabled). The functionality of the IOMMU is different
depending on whether it's running on a bare metal PowerNV system or in
a virtual environment (PowerVM LPAR or KVM/QEMU).  DPDK currently
supports the IOMMU found on PowerNV platforms, sPAPRv2, so IOVA=VA
mode can be enabled when the correct platform is detected.

The POWER IOMMU type can't be detected through mechanisms such as
parsing files in the /sys hierarchy like x86_64 systems so the
/proc/cpuinfo file is parsed to determine whether Linux is running
on bare metal (i.e. PowerNV) or in a virtual environment (KVM/QEMU).

Signed-off-by: David Christensen <drc@linux.vnet.ibm.com>