machine: add support for the X9 Pro smartwatch

This is a Chinese smart watch sold on AliExpress etc. It is based on a
nrf52832. More information:
https://github.com/curtpw/nRF5x-device-reverse-engineering/tree/master/X9-nrf52832-activity-tracker

targets/x9pro.json

@@ -0,0 +1,8 @@
+{
+	"inherits": ["nrf52"],
+	"build-tags": ["x9pro"],
+	"flash-method": "openocd",
+	"flash-command": "nrfjprog -f nrf52 --sectorerase --program {hex} --reset",
+	"openocd-interface": "jlink",
+	"openocd-transport": "swd"
+}