dts: bindings: Adapt nordic,bt-hci-sdc to recent upstream changes

`bt-hci-bus` property values are now specified in lowercase and without
the `BT_HCI_BUS_` prefix.

Signed-off-by: Andrzej Głąbek <[email protected]>

Author: anangl

dts/bindings/bluetooth/nordic,bt-hci-sdc.yaml

@@ -8,4 +8,4 @@ properties:
   bt-hci-name:
     default: "SDC"
   bt-hci-bus:
-    default: "BT_HCI_BUS_VIRTUAL"
+    default: "virtual"