boards: arm: sparkfun_pro_micro_rp2040: Support JTAG

Add SWD debugger support for this board.  Although there is not a debug
header, the SWD pins are accessible and the adaptor is not that difficult
to wire up.  This will allow for debugging beyond just flashing, that the
UF2 programming setup allows for.

Signed-off-by: David Brown <david.brown@linaro.org>