ipq806x: add ethernet (stmmac) support

We forgot to enable the stmmac driver for 4.1 kernel, so ethernet
interfaces don't show-up on this kernel.

Signed-off-by: Mathieu Olivari <mathieu@codeaurora.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46559 3c298f89-4303-0410-b956-a3cf2f4a3e73