net: Add depends on OF_NET for LiteX's LiteETH

Current settings may produce a build error when
CONFIG_OF_NET is disabled. The CONFIG_OF_NET controls
a headfile <linux/of.h> and some functions
 in <linux/of_net.h>.

Signed-off-by: Slark Xiao <slark_xiao@163.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>