Add Runtime PM support to the sh_eth driver.

The clock to the ethernet hardware block will be
enabled as long as the network device is up.
Signed-off-by: Magnus Damm <damm@opensource.se>
Tested-by: Kuninori Morimoto <morimoto.kuninori@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>