l1tol2: put a flow Q on the exits (#606)

This Xbar connects the largest components in the design; the cores
and the L2 banks. We already have a full buffer on the core side.
However, the valid path going to the L2 comes back as a ready path.
Putting a flow Q also on the outputs of the l1tol2 cuts this path
in half at no cost to IPC.