Make fasm2frames enable STEPDOWN for unbonded IOBs

[mkurc-ant]

Currently fasm2frames looks for a *STEPDOWN feature in each used IOB tile. When its found its replicated for the rest of the IO bank of that tile. However, this is done basing on package_pinmap.csv which holds only bonded IOBs so the feature is not replicated for the unbonded ones.

Vivado emits *STEPDOWN (whenever necessary) for the whole IO bank regardless of whether IOBs are bonded or not. fasm2frames should too.