nrf: add support for the PineTime64 devkit

This smartwatch doesn't have an on-board debugger, so I picked the one I
was using while getting this smartwatch to run Go programs (the J-Link
EDU Mini).

targets/pinetime-devkit0.json

@@ -0,0 +1,8 @@
+{
+	"inherits": ["nrf52"],
+	"build-tags": ["pinetime_devkit0"],
+	"flash-method": "openocd",
+	"flash-command": "nrfjprog -f nrf52 --sectorerase --program {hex} --reset",
+	"openocd-interface": "jlink",
+	"openocd-transport": "swd"
+}