samples: Bluetooth: bap_broadcast_source nRF5340 audio dk + nRF21540 FEM

Added overlay file to support nRF5340 audio dk + nRF21540
FEM.

Build the hci_ipc with corresponding overlay file to use the
nRF21540 FEM with the bap_broadcast_source sample on nRF5340
audio kit.

Use the below commandline:
$ west build -b nrf5340_audio_dk/nrf5340/cpuapp --sysbuild \
  samples/bluetooth/bap_broadcast_source \
  -DDTC_OVERLAY_FILE= \
  boards/nrf5340_audio_dk_nrf5340_cpuapp_nrf21540_ek.overlay \
  -Dhci_ipc_DTC_OVERLAY_FILE= \
  boards/nrf5340_audio_dk_nrf5340_cpunet_nrf21540_ek.overlay

Related to commit 35ad6530 ("samples: Bluetooth:
hci_uart/hci_ipc: CI coverage for nRF53+nRF21 FEM").

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>