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/snapper9g20_defconfig b/configs/snapper9g20_defconfig
index eb34507..c6eca22 100644
--- a/configs/snapper9g20_defconfig
+++ b/configs/snapper9g20_defconfig
@@ -49,4 +49,5 @@
 # CONFIG_SYS_NAND_USE_FLASH_BBT is not set
 CONFIG_NAND_ATMEL=y
 CONFIG_MACB=y
+CONFIG_RMII=y
 CONFIG_ATMEL_USART=y