ar71xx: add support for Onion Omega

This patch adds support for the Onion Omega.

https://onion.io/omega

Signed-off-by: L. D. Pinney <ldpinney@gmail.com>
Acked-by: Boken Lin <bl@onion.io>
Tested-by: Jacky Huang <huangfangcheng@163.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46458 3c298f89-4303-0410-b956-a3cf2f4a3e73