clk: add call for nodes to get the programmed/decided frequency passed back

Summary:

The existing call can only really be used for a node wishing to
configure its parent, but as we don't pass in a pointer to the freq,
we can't set it to what it would be for a DRY_RUN pass.

So for clock nodes that wish to try setting parent frequencies to see
which would be the best for its own target frequency, we really do need
a way to call in and pass in a flag /and/ a pointer to freq so it can be
updated for us as the clock tree is recursed through.

Reviewers: manu

Approved by: manu

Subscribers: imp

Differential Revision: https://reviews.freebsd.org/D33445