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>

drivers/net/ethernet/litex/Kconfig

@@ -17,6 +17,7 @@ if NET_VENDOR_LITEX
 
 config LITEX_LITEETH
 	tristate "LiteX Ethernet support"
+	depends on OF_NET
 	help
 	  If you wish to compile a kernel for hardware with a LiteX LiteEth
 	  device then you should answer Y to this.