staging: pi433: overlay: Fix Broadcom vendor prefix

checkpatch.pl says:

    WARNING: DT compatible string "bcm,bcm2708" appears un-documented -- check ./Documentation/devicetree/bindings/

The vendor prefix of Broadcom Corporation is "brcm", not "bcm".
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200221122133.32024-2-geert+renesas@glider.beSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/pi433/Documentation/devicetree/pi433-overlay.dts

@@ -3,7 +3,7 @@
 /plugin/;
 
 / {
-	compatible = "bcm,bcm2835", "bcm,bcm2708", "bcm,bcm2709";
+	compatible = "brcm,bcm2835", "brcm,bcm2708", "brcm,bcm2709";
 
 	fragment@0 {
 		target = <&spi0>;