Improve mpl2 so that sky130hd/chameleon doesn't have to use manual macro placement

[AcKoucher]

Description

#5256 Fixed the crash in mpl2; #5255 Solved the problem of mpl2 not completing due SA not finding a valid tiling for the root.

However, even though mpl2 flow finishes there's a huge deterioration in WL.

Suggested Solution

Use only constrained pin data for IO Pins in mpl2 flow so that hierarchical macro placement SA can find a better floorplan.

Additional Context

No response