ath79: apply Engenius EAP600 style to OpenMesh MR600 RGMII cfg

ath79: apply Engenius EAP600 style to OpenMesh MR600 RGMII cfg

The OpenMesh MR600 is a modified version of the EAP600 family. These
devices are shipped with an AR803x PHY and had various problems with the
delay configuration in ar71xx. These problems are now in the past [1] and
parts of the delay configuration should now be done in the PHY only.

Just switch to the configuration of the EAP600 to have an already well
tested configuration for ath79 with the newer kernel versions.

[1] https://github.com/openwrt/openwrt/pull/3505#issuecomment-716050292

Reported-by: Michael Pratt <mcpratt@pm.me>
Signed-off-by: Sven Eckelmann <sven@narfation.org>