ARM: dts: BCM53573: Relicense Tenda AC9 file to the GPL 2.0+ / MIT

This matches licensing used by most of BCM5301X files and is preferred as:
1) GPL 2.0+ makes it clearly compatible with Linux kernel
2) MIT is also permissive but preferred over ISC

This file was fully developed by me.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

arch/arm/boot/dts/bcm47189-tenda-ac9.dts

+// SPDX-License-Identifier: GPL-2.0-or-later OR MIT
 /*
  * Copyright (C) 2016 Rafał Miłecki <rafal@milecki.pl>
- *
- * Licensed under the ISC license.
  */
 
 /dts-v1/;