pb_type_xml: added docstrings and simplyfied get_site_pin_wire logic
pb_type_xml: added docstrings and simplyfied get_site_pin_wire logic

Signed-off-by: Alessandro Comodi <622632e70fd6771cf2cd943f69e5e51ab66adef8@antmicro.com>

SymbiFlow/symbiflow-arch-defs,SymbiFlow/symbiflow-arch-defs