Add support for pin drive strength

[Tobias-DG3YEV]

Please add support for pin drive strength.
This is set with "set_property DRIVE" in the .xdc file.

Currently nextpnr-xilinx only give an assertion, when it finds such entry in the xdc.

what(): Assertion failure: !is_string (/home/tobiw/nextpnr/tmp/nextpnr-xilinx/common/nextpnr.h:350) make: *** [Makefile:29: spec2g4.fasm] Aborted

[cestrauss]

I traced it back to

nextpnr-xilinx/xilinx/pack_io_xc7.cc

Line 458 in 159aaa8

auto drive = drive_attr->second.as_int64();

It seems the Property value is stored as a string, not as an integer. The as_int64() method does not actually do an automatic conversion from string to integer, raising an exception instead.