cmd: mdio/mii: add Kconfig help and allow break dependency

* Add Kconfig help describing the purpose of each command.
* Add CONFIG_CMD_MDIO so it could be selected individually, as
  it doesn't depend on the mii command.
* Add Kconfig imply to mii to automatically select the mdio
  command.

Signed-off-by: Ramon Fried <rfried.dev@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index bc8318d..1e4cf14 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1473,8 +1473,22 @@
 
 config CMD_MII
 	bool "mii"
+	imply CMD_MDIO
 	help
-	  Enable MII utility commands.
+	  If set, allows 802.3(clause 22) MII Management functions interface access
+	  The management interface specified in Clause 22 provides
+	  a simple, two signal, serial interface to connect a
+	  Station Management entity and a managed PHY for providing access
+	  to management parameters and services.
+	  The interface is referred to as the MII management interface.
+
+config CMD_MDIO
+	bool "mdio"
+	depends on PHYLIB
+	help
+	  If set, allows Enable 802.3(clause 45) MDIO interface registers access
+	  The MDIO interface is orthogonal to the MII interface and extends
+	  it by adding access to more registers through indirect addressing.
 
 config CMD_PING
 	bool "ping"