mem-cache: Selectively clear downstream pending

mem-cache: Selectively clear downstream pending

Until now, all deferred targets of an MSHR would be promoted together
as soon as the targets were serviced. When we promote deferred targets
we also clear the downstreamPending flag.

Due to the way we handle cache clean operations we might need to
promote only deferred targets that don't require writable, leaving
some targets as deferred. To allow for partial target promotion, this
change adds support for clearing the downstreamPending only for a
subset of a TargetsList.

Change-Id: Id06953643ba9a975ebacc76ac10215441e264e74
Reviewed-on: https://gem5-review.googlesource.com/11015
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>