Convert CONFIG_RMII to Kconfig

This converts the following to Kconfig:
   CONFIG_RMII

Cc: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/devkit3250_defconfig b/configs/devkit3250_defconfig
index 2a24e01..df8d0d1 100644
--- a/configs/devkit3250_defconfig
+++ b/configs/devkit3250_defconfig
@@ -70,6 +70,7 @@
 CONFIG_PHY_ADDR_ENABLE=y
 CONFIG_PHY_ADDR=31
 CONFIG_PHY_SMSC=y
+CONFIG_RMII=y
 CONFIG_SPECIFY_CONSOLE_INDEX=y
 CONFIG_CONS_INDEX=5
 CONFIG_SYS_NS16550=y