Convert CONFIG_MII et al to Kconfig

This converts the following to Kconfig:
   CONFIG_MII
   CONFIG_DRIVER_TI_EMAC

Signed-off-by: Adam Ford <aford173@gmail.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/configs/stm32f746-disco_defconfig b/configs/stm32f746-disco_defconfig
index aab5075..8ff6404 100644
--- a/configs/stm32f746-disco_defconfig
+++ b/configs/stm32f746-disco_defconfig
@@ -52,6 +52,7 @@
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_DM_ETH=y
 CONFIG_ETH_DESIGNWARE=y
+CONFIG_MII=y
 # CONFIG_PINCTRL_FULL is not set
 CONFIG_SPI=y
 CONFIG_DM_SPI=y