dts: add binding for nrf21540-fem-spi

The nRF21540 radio front-end-module can optionally be controlled by a
SPI interface.  Add a binding and property to link the SPI bus
configuration data to the FEM node.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

dts/bindings/net/wireless/nordic,nrf21540-fem-spi.yaml

@@ -0,0 +1,10 @@
+# Copyright (c) 2020 Nordic Semiconductor ASA
+# SPDX-License-Identifier: Apache-2.0
+
+
+description: >
+    This is a SPI device interface to the nRF21540 Radio Front-End module
+
+compatible: "nordic,nrf21540-fem-spi"
+
+include: spi-device.yaml

dts/bindings/net/wireless/nordic,nrf21540-fem.yaml

@@ -29,6 +29,13 @@ properties:
         required: false
         description: >
             Gpio of the SOC controlling ANT-SEL pin of the nRF21540
+    spi-if:
+        type: phandle
+        required: false
+        description: >
+            Reference to the nordic,nrf21540-fem-spi SPI bus interface.
+
+            This must be present to support SPI control of the FEM.
     tx-en-settle-time-us:
         type: int
         default: 11