ar71xx: enable SGMII fixup on Mikrotik wAP AC

ar71xx: enable SGMII fixup on Mikrotik wAP AC

fixes intermittent loss of connectivity on 1Gbit port, with log message:
> 803x_aneg_done: SGMII link is not ok

Thanks to David Bauer for pointing me in the right direction.
I just had to figure out the right bus_id, which you find in this log:
> ag71xx ag71xx.1: connected to PHY at gpio-1:00 [uid=004dd074,
  driver=Atheros 8031 ethernet]

Fixes FS#2236

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>
[Wrapped commit message - Fixed whitespace erors]
Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 2a7519e29d6cbbe1eb73623ef35add0ef596fb5f)