mem: Add StreamID and SubstreamID

mem: Add StreamID and SubstreamID

This patch adds StreamID and SubstreamID to Request. These fields can
be used by a SMMU/IOMMU model to pick up the correct translation
context for each request and they correspond to an ASID in a device.
For this reason they have been merged together with the request asid
in a union, so that a cpu will set the asid and a device will set
the Stream and Substream ID.

Change-Id: Iac2b5a1ba9c6598ee7635c30845dc68ba6787c34
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/12187
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>