mb/google/volteer/variants: Disable non-existing BT PCI interface and add BT flag

Remove the CNVi BT PCI config and add BT flag. There is no PCI host interface
in this version of CNVi.
TEST: BT is checked using 'lsusb -d 8087:0026' from OS to make sure BT is
enumerated.

Change-Id: Ic700021d7a09be63ffc2715f31992257e2e893af
Signed-off-by: Cliff Huang <cliff.huang@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50898
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/volteer/variants/baseboard/devicetree.cb

@@ -295,6 +295,8 @@ chip soc/intel/tigerlake
 
 	register "tcc_offset" = "10"     # TCC of 90
 
+	register "CnviBtCore" = "true"
+
 	register "CnviBtAudioOffload" = "true"
 
 	# Intel Common SoC Config
@@ -467,7 +469,6 @@ chip soc/intel/tigerlake
 		end
 		device ref gna on end
 		device ref north_xhci on end
-		device ref cnvi_bt on end
 		device ref south_xhci on end
 		device ref shared_ram on end
 		device ref cnvi_wifi on