ipq40xx: add support for FRITZ!Box 7520

ipq40xx: add support for FRITZ!Box 7520

This model, also know as "1&1 HomeServer", shares the same features as 7530.

The vendor firmware has artificial software limitations: only 2 of the 4
LAN-Ports are GBit, and the USB-Host is only v2.0.

With OpenWrt, USB is already working at v3.0.

Signed-off-by: Andre Heider <a.heider@gmail.com>
(updated commit message to reflect current state)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
(cherry picked from commit cb6f4be13703f0224fc462caaeac14e725c72986)