net: Add sunxi (Allwinner) wemac driver

This patch adds support for the WEMAC, the ethernet controller included
in the Allwinner A10 SoC. It will get used in the upcoming A10 board
support.

From: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Henrik Nordstrom <henrik@henriknordstrom.net>