ar71xx: rb91x: fix ethernet packet loss issues

The RB91x boards are suffering from ethernet packet loss after a cold
boot. The cause of the problem is that the AR8035 PHYs requires special
register settings to work reliably on these boards.

Enable the RGMII TX, RX delays and disable SmartEE functionality of
the AR8035 PHYs. Also enable the RXD delay in the ETH_CFG register
to fix the issue.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 40509