Convert CONFIG_MII_INIT to Kconfig

This converts the following to Kconfig:
   CONFIG_MII_INIT

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/M5373EVB_defconfig b/configs/M5373EVB_defconfig
index c59ba94..38d2002 100644
--- a/configs/M5373EVB_defconfig
+++ b/configs/M5373EVB_defconfig
@@ -17,6 +17,7 @@
 CONFIG_CMD_NAND=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CMD_MII=y
+CONFIG_MII_INIT=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_CACHE=y
 CONFIG_CMD_DATE=y