samples: usb: wpanusb: add MCR20A overlay config

Add support for NXP MCR20A 802.15.4 module as an overlay config file.

Use this file like so:
cmake -DBOARD=usb_kw24d512 \
      -DOVERLAY_CONFIG=overlay-mcr20a.conf ..

Signed-off-by: Michael Scott <mike@foundries.io>
diff --git a/samples/net/wpanusb/overlay-mcr20a.conf b/samples/net/wpanusb/overlay-mcr20a.conf
new file mode 100644
index 0000000..7cb05a0
--- /dev/null
+++ b/samples/net/wpanusb/overlay-mcr20a.conf
@@ -0,0 +1,2 @@
+CONFIG_IEEE802154_MCR20A=y
+CONFIG_NET_APP_IEEE802154_DEV_NAME="mcr20a"