Add a few SPDX tags

Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43151 3c298f89-4303-0410-b956-a3cf2f4a3e73
ar71xx: add support for Netgear WNR2000v4