[Arch] Add an example definition of external bitstream to openfpga arch with soft adder
<!-- Binding operating pb_type to physical pb_type -->
<!-- Binding operating pb_types in mode 'arithmetic' -->
<pb_type name="clb.fle[arithmetic].soft_adder.adder_lut4" physical_pb_type_name="clb.fle[physical].fabric.frac_logic.frac_lut4" mode_bits="1">
+ <external_bitstream source="eblif" attribute=".param" keyword_to_ignore="LUT"/>
</pb_type>
<pb_type name="clb.fle[arithmetic].soft_adder.ff" physical_pb_type_name="clb.fle[physical].fabric.ff"/>
<pb_type name="clb.fle[arithmetic].soft_adder.carry_follower" physical_pb_type_name="clb.fle[physical].fabric.frac_logic.carry_follower"/>
