ar71xx: fix ethernet packet loss issues on OM5P-AN

The OM5P-AN boards are suffering from ethernet packet loss when booting with
some active POE setups or when switching to Fast Ethernet when previously
booted with Gigabit ethernet attached.

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 and RDV
delay in the ETH_CFG register to fix the issue.

Signed-off-by: Sven Eckelmann <sven@open-mesh.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45438 3c298f89-4303-0410-b956-a3cf2f4a3e73