gateware.platform: add apollo_gateware_phy to LUNAApolloPlatform (#269)

Adds the apollo_gateware_phy property to LUNAApolloPlatform to specify
the preferred USB PHY for Apollo gateware.

When the platform does not share a port with Apollo, it defaults to
default_usb_connection.

luna/gateware/platform/core.py

@@ -119,6 +119,17 @@ def port_sharing(self, phy_name):
         sharing = getattr(self, "apollo_port_sharing", {})
         return sharing.get(phy_name, None)
 
+
+    @property
+    def apollo_gateware_phy(self):
+        """ Returns the USB PHY for Apollo gateware. """
+
+        sharing = getattr(self, "apollo_port_sharing", {})
+        if len(sharing) == 0:
+            return self.default_usb_connection
+        return list(sharing.keys())[0]
+
+
     def toolchain_program(self, products, name):
         """ Programs the relevant LUNA board via its sideband connection. """