kernel: switch to upstream

This driver is upstream since 2012 (commit: a8e510f682f), so switch to
it and remove own sources. Also place it under "Network Devices" menu
together with other phy drivers.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45269 3c298f89-4303-0410-b956-a3cf2f4a3e73