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>