boards: nrf21540dk_nrf52840: Assign nRF21540 FEM to Radio device

nRF21540 DK has nRF21540 Front End Module connected with
nRF52840 CPU, assign the FEM as default to Radio device.

Relates to commit 7ff74e0a ("Bluetooth: controller:
support FEMs via devicetree")

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