ar71xx: fix switch probing on kernel 4.14

ar71xx: fix switch probing on kernel 4.14

The bump to 4.14 changed the way mdio probes behind switches.

While the board_info is added to the list, the code that actually inserted
the list info into the phydev structure was missing.

This resulted in non-working ethernet ports.

Re-add it to fix switch probing.
This mimics the exact behaviour as it was in kernel 4.9.

Before:

[    1.066007] switch0: Atheros AR8327 rev. 4 switch registered on ag71xx-mdio.0
[    1.073409] Atheros AR8216/AR8236/AR8316: probe of ag71xx-mdio.0:00 failed with error -22
[    1.102455] libphy: ag71xx_mdio: probed
[    1.737938] ag71xx ag71xx.0: connected to PHY at ag71xx-mdio.0:00 [uid=004dd034, driver=Generic PHY]
[    1.747994] eth0: Atheros AG71xx at 0xb9000000, irq 4, mode:RGMII
[    2.377642] ag71xx-mdio.1: Found an AR934X built-in switch
[    2.429938] eth1: Atheros AG71xx at 0xba000000, irq 5, mode:GMII

After:

[   11.163357] libphy: Fixed MDIO Bus: probed
[   11.319898] libphy: ag71xx_mdio: probed
[   11.360844] switch0: Atheros AR8327 rev. 4 switch registered on ag71xx-mdio.0
[   12.447398] libphy: ag71xx_mdio: probed
[   13.077402] ag71xx ag71xx.0: connected to PHY at ag71xx-mdio.0:00 [uid=004dd034, driver=Atheros AR8216/AR8236/AR8316]
[   13.088989] eth0: Atheros AG71xx at 0xb9000000, irq 4, mode:RGMII
[   13.717716] ag71xx-mdio.1: Found an AR934X built-in switch
[   13.769990] eth1: Atheros AG71xx at 0xba000000, irq 5, mode:GMII

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
(backported from 42f158314e011293ffdaeab5870b19110f4e2e9d)