nrf: support debugging the PCA10056

This change adds support for `tinygo gdb` on the PCA10056.

The board is normally flashed with the MSD programmer. Debugging needs a
real debugger interface however, which is relatively simple by just
adding the right OpenOCD configuration.

targets/pca10056.json

@@ -4,5 +4,7 @@
 	"flash-method": "command",
 	"flash-command": "nrfjprog -f nrf52 --sectorerase --program {hex} --reset",
 	"msd-volume-name": "JLINK",
-	"msd-firmware-name": "firmware.hex"
+	"msd-firmware-name": "firmware.hex",
+	"openocd-interface": "jlink",
+	"openocd-transport": "swd"
 }