adding new uf2 target for PCA10056 (#3765)

targets: adding new uf2 target for PCA10056

Author: jpconstantineau

targets/pca10056-s140v6-uf2.json

@@ -0,0 +1,6 @@
+{
+    "inherits": ["nrf52840", "nrf52840-s140v6-uf2"],
+    "build-tags": ["pca10056"],
+    "serial-port": ["239a:0x0029"],
+    "msd-volume-name": ["NRF52BOOT"]
+}