[][kernel][common][eth][Add EEE support for GDM]

[Description]
Add EEE support for GDM.
  - ethtool --set-eee eth1 tx-lpi on tx-timer 1
  - ethtool --show-eee eth1

If without this patch, user can't use ethtool to enable
MAC/PHY EEE capability.

[Release-log]
N/A


Change-Id: I624fbefba33fa2d75bc10efec0531b96f7d8034a
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6829459
2 files changed