MIPS: BCM63xx: switch to SPDX license identifier

Use SPDX license indentifier instead of local reference to COPYING.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>