ARM: dts: BCM5301X: Relicense Asus RT-AC87U file to the GPL 2.0+ / MIT

ARM: dts: BCM5301X: Relicense Asus RT-AC87U file to the GPL 2.0+ / MIT

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

This file were created and ever touched by a group of three people only:
Álvaro, Hauke and me.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Álvaro Fernández Rojas <noltari@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>