drivers: net: davinci_mdio: add pm runtime callbacks

Add PM runtime .runtime_suspend()/.runtime_resume() callbacks and
perform Davinci MDIO enabling/disabling from these callbacks. This
allows to reuse pm_runtime_force_suspend/resume() APIs during System
suspend and required for further implementation of PM runtime
autosuspend.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>