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.be


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>