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/axm_defconfig b/configs/axm_defconfig
index 3373e62..c4ef342 100644
--- a/configs/axm_defconfig
+++ b/configs/axm_defconfig
@@ -83,6 +83,7 @@
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_MACB=y
+CONFIG_RMII=y
 CONFIG_PINCTRL=y
 CONFIG_PINCTRL_AT91=y
 CONFIG_SPECIFY_CONSOLE_INDEX=y