ath79: use downstream ag71xx for Kernel 5.4

ath79: use downstream ag71xx for Kernel 5.4

The ag71xx driver from Linux 5.4 currently has various shortcomings
when used with OpenWrt compared to our downstream version.

For example, the upstream driver does not support modifying the ethernet
clock and configuring RGMII delays on the MAC side.

While we should certainly switch to the upstream driver, the amount of
necessary patches would make it cumbersome to work with. It's also
highly likely we won't be able to finish patching the upstream driver in
time for a Linux 5.4 release.

Tested on Siemens WS-AP3610.

CC: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David Bauer <mail@david-bauer.net>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>