nrf52840: add build tags for SoftDevice support

The SoftDevice should already be installed on these chips. Adding the
right build tags makes them work with the bluetooth package.

I did not change the HasLowFrequencyCrystal property: all these boards
use the MDBT50Q which appears to include a low-frequency oscillator.
That is, I tested the ItsyBitsy nRF52840 with the property set to true
and advertisement worked just fine.

targets/clue_alpha.json

@@ -1,6 +1,6 @@
 {
     "inherits": ["nrf52840"],
-    "build-tags": ["clue_alpha","nrf52840_reset_uf2"],
+    "build-tags": ["clue_alpha","nrf52840_reset_uf2", "softdevice", "s140v6"],
     "flash-1200-bps-reset": "true",
     "flash-method": "msd",
     "msd-volume-name": "FTHR840BOOT",

targets/feather-nrf52840.json

@@ -1,6 +1,6 @@
 {
     "inherits": ["nrf52840"],
-    "build-tags": ["feather_nrf52840","nrf52840_reset_uf2"],
+    "build-tags": ["feather_nrf52840","nrf52840_reset_uf2", "softdevice", "s140v6"],
     "flash-1200-bps-reset": "true",
     "flash-method": "msd",
     "msd-volume-name": "FTHR840BOOT",

targets/itsybitsy-nrf52840.json

@@ -1,6 +1,6 @@
 {
     "inherits": ["nrf52840"],
-    "build-tags": ["itsybitsy_nrf52840","nrf52840_reset_uf2"],
+    "build-tags": ["itsybitsy_nrf52840","nrf52840_reset_uf2", "softdevice", "s140v6"],
     "flash-1200-bps-reset": "true",
     "flash-method": "msd",
     "msd-volume-name": "ITSY840BOOT",