ar71xx: improve support for TP-Link CPE510 v2

ar71xx: improve support for TP-Link CPE510 v2

This fixes commit bae927c551fd ("ar71xx: add support for TP-LINK CPE510
V2.0") where the support for this device wasn't optimal.

Device support for the CPE510v2 so far has been a hack to enable
flashing with CPE510v1 images. Those even have different hardware (e.g.
additional ethernet port).

With this patch, we provide proper support for this device in ar71xx.

Installation:
- Flash factory image through stock firmware WEB UI or through TFTP
- To get to TFTP recovery just hold reset button while powering on
  for around 4-5 seconds and release.
- Rename factory image to recovery.bin
- Stock TFTP server IP: 192.168.0.100
- Stock device TFTP address: 192.168.0.254

Fixes: bae927c551fd ("ar71xx: add support for TP-LINK CPE510 V2.0")
Signed-off-by: Andrew Cameron <apcameron@softhome.net>
[Rebased onto revert commit, changed comments in mach-cpe510.c,
changed commit title and description, fixed eth0 MAC address,
removed eth1 initialization]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
[squashed revert, added fixes tag]
Signed-off-by: Petr Štetiar <ynezz@true.cz>