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/corvus_defconfig b/configs/corvus_defconfig
index c7a59c7..3dd55c0 100644
--- a/configs/corvus_defconfig
+++ b/configs/corvus_defconfig
@@ -75,6 +75,7 @@
 CONFIG_SYS_NAND_U_BOOT_LOCATIONS=y
 CONFIG_SYS_NAND_U_BOOT_OFFS=0x20000
 CONFIG_MACB=y
+CONFIG_RMII=y
 CONFIG_ATMEL_USART=y
 CONFIG_USB=y
 CONFIG_USB_EHCI_HCD=y