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/Makefile b/cmd/Makefile
index e87c2f1..3ac7104 100644
--- a/cmd/Makefile
+++ b/cmd/Makefile
@@ -89,9 +89,7 @@
 obj-$(CONFIG_CMD_IO) += io.o
 obj-$(CONFIG_CMD_MFSL) += mfsl.o
 obj-$(CONFIG_CMD_MII) += mii.o
-ifdef CONFIG_PHYLIB
-obj-$(CONFIG_CMD_MII) += mdio.o
-endif
+obj-$(CONFIG_CMD_MDIO) += mdio.o
 obj-$(CONFIG_CMD_MISC) += misc.o
 obj-$(CONFIG_CMD_MMC) += mmc.o
 obj-$(CONFIG_MP) += mp.o