ARM: dts: BCM5301X: Relicense DTS files I created to the ISC

It's preferred to have DT source files licensed under BSD compatible
license. All new BCM5301X DTS files use ISC so let's also relicense old
ones to it.

Except for me only Hauke was ever touched these files in his commit
9faa5960 ("ARM: BCM5301X: add NAND flash chip description") and
commit bb1d8fba

 ("ARM: BCM5301X: add NAND flash chip description for
Asus RT-AC87U").

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