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/gurnard_defconfig b/configs/gurnard_defconfig
index b90437f..47678e6 100644
--- a/configs/gurnard_defconfig
+++ b/configs/gurnard_defconfig
@@ -49,6 +49,7 @@
 CONFIG_NAND_ATMEL=y
 CONFIG_ATMEL_NAND_HWECC=y
 CONFIG_MACB=y
+CONFIG_RMII=y
 CONFIG_ATMEL_USART=y
 CONFIG_TIMER=y
 CONFIG_ATMEL_PIT_TIMER=y