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/drivers/net/Kconfig b/drivers/net/Kconfig
index 71e0cba..d63c0a9 100644
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -535,6 +535,11 @@
 	help
 	  Enable support of the Media-Independent Interface (MII)
 
+config RMII
+	bool "Enable RMII"
+	help
+	  Enable support of the Reduced Media-Independent Interface (MII)
+
 config PCNET
 	bool "AMD PCnet series Ethernet controller driver"
 	help