Add Microsemi PDC format output

[CGrassin]

Support for Microsemi's Physical Design Constraint File (*.pdc file extension) for Libero SoC (for the following FPGA family: PolarFire, SmartFusion2, IGLOO2, RTG4, SmartFusion, IGLOO, ProASIC3, Fusion).

Sample:

set_io LED1 -pinname J16 -fixed yes -DIRECTION OUTPUT

[g0t00]

That seems to generally be implemented.
But for ProAsic 3 there seems to be a bogus pin to be detected.

set_io pin_ -pinname  -fixed yes

Also some power pins seem to be detected correctly and commented out, but some are not :(