ar71xx: add support for tp-link wr740n v5.0 (EU)

TP-Link ships wr740n v5.0 to Romania (so probably everywhere except
China) with 4 MB flash and 32 MB memory (confirmed by their local
support). This patch adds support for the v5.0 clone of v4, i just
tested it on my own v5.0 router and it works.

Signed-off-by: Daniel Petre <daniel.petre@posteo.net>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46115 3c298f89-4303-0410-b956-a3cf2f4a3e73