ar71xx: add support for TL-WR741ND v5

This device is identical to the TL-WR740N v5, it even uses the same HWID (which
wasn't the case for older TL-WR741ND revisions).

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46238 3c298f89-4303-0410-b956-a3cf2f4a3e73