Convert CONFIG_DRIVER_TI_EMAC_USE_RMII to Kconfig

This converts the following to Kconfig:
   CONFIG_DRIVER_TI_EMAC_USE_RMII

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/drivers/net/ti/Kconfig b/drivers/net/ti/Kconfig
index ecf642d..f2dbbd0 100644
--- a/drivers/net/ti/Kconfig
+++ b/drivers/net/ti/Kconfig
@@ -14,6 +14,12 @@
 	help
 	   Support for davinci emac
 
+config DRIVER_TI_EMAC_USE_RMII
+	depends on DRIVER_TI_EMAC
+	bool "Use RMII"
+	help
+	  Configure the TI EMAC driver to use RMII
+
 config DRIVER_TI_KEYSTONE_NET
 	bool "TI Keystone 2 Ethernet"
 	help