ARM: dts: BCM5301X: Relicense most DTS files to the GPL 2.0+ / MIT

These files were created and ever touched by a group of three people
only: Dan, Hauke and me. They were licensed under GNU/GPL or ISC.

Introducing and discussing SPDX-License-Identifier resulted in a
conclusion that ISC is a not recommended license (see also a
license-rules.rst). Moveover an old e-mail from Alan Cox was pointed
which explained that dual licensing is a safer solution than depending
on a common compatibility belief.

This commit switches most of BCM5301X DTS files to dual licensing using:
1) GPL 2.0+ to make sure they are compatible with Linux kernel
2) MIT to allow sharing with more permissive projects
Both licenses belong to the preferred ones (see LICENSES/preferred/).

An attempt to relicense remaining files will be made separately and will
require approve from more/other developers.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Dan Haab <dan.haab@luxul.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>