fpga/arty: add TinyRocketArtyBScanConfig, JTAG DTM over the FPGA TAP

[disdi]

The Arty A7-35T harness brings the SoC's JTAG DTM out on PMOD, so debugging needs a second JTAG adapter even though the board's USB cable already drives the Artix-7 TAP. Add WithArtyBScanJTAGHarnessBinder, which routes the JTAGPort through fpga-shells' JTAGTUNNEL (BSCANE2 on USER4, TCK gated by SEL via BUFGCE), and TinyRocketArtyBScanConfig that selects it in place of the PMOD binder while keeping the stock WithJtagDTM.

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• Bug fix
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• RTL change
• Software change (RISC-V software)
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