deps: Re-pin to released versions once the common-cells v2 chain lands

[DanielKellerM]

0.7.0 ships with four of six direct dependencies pinned to raw git revisions rather than released versions, and common_cells on a beta.

dependency	pinned to	upstream state
common_cells	v2.0.0-beta.3	tagged, and master tip
axi_stream	master tip	no tag at that revision
axi	devel tip	no tag at that revision
obi	common-cells-v2	open PR obi#36
apb	common-cells-v2	open PR apb#19

This is a deliberate decision for 0.7.0, not an oversight. The whole set is the common-cells v2 migration, which cannot be pinned to releases until the chain lands upstream.

Nothing can rot: every pinned revision is reachable from a permanent ref, either a tag, a mainline branch, or a refs/pull/N/head which GitHub retains after branch deletion. v0.7.0 therefore stays buildable regardless of what happens to those branches.

What does need revisiting is that obi and apb are pinned to unmerged PR branches. If those PRs are revised before merging, iDMA is pinned to an intermediate state that never became the released version, and the pins will need refreshing rather than just relabelling.

To do once upstream lands

• common_cells - move off the beta to the v2 release when tagged
• obi - re-pin to a release once obi#36 merges
• apb - re-pin to a release once apb#19 merges
• axi - re-pin to a release carrying the common-cells v2 support
• axi_stream - re-pin to a release if one is cut

Version impact: re-pinning to identical code is a patch; if resolution changes materially it is a 0.x minor, since in 0.x the minor is the breaking axis.