net: ethernet: davinci_emac: add pm_runtime support

Add pm_runtime support to the TI Davinci EMAC driver.

CC: Sekhar Nori <nsekhar@ti.com>
CC: Kevin Hilman <khilman@ti.com>
Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>