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/configs/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index f235978..833a55c 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -63,6 +63,7 @@
 CONFIG_DM_ETH=y
 CONFIG_MII=y
 CONFIG_DRIVER_TI_EMAC=y
+CONFIG_DRIVER_TI_EMAC_USE_RMII=y
 CONFIG_PINCTRL=y
 CONFIG_PINCTRL_SINGLE=y
 CONFIG_DM_PMIC=y