[SRU][Zesty] Support SMMU passthrough using the default domain

[Manoj Iyer]

This patch series adds support for SMMU passthrough using the default domain,
the patch series implements a kernel command-line option iommu.passthrough=0/1
1 - Bypass the IOMMU for DMA, 0 - Use IOMMU translation for DMA. This feature
is desired on both Qualcomm Centriq 2400 and Thunder-X platforms.

The bug report http://bugs.launchpad.net/bugs/1688158 has detailed test results
from Qualcomm demonstrating performance improvements using passthrough. I have
also tested the kernel on Thundex-X for any regressions, for both boot test and
pci passthrough.

Please review and consider for SRU.

Cheers
Manoj Iyer