sh_eth: enclose PM code into #ifdef CONFIG_PM

Put '#ifdef CONFIG_PM' around sh_eth_runtime_nop() and 'sh_eth_dev_pm_ops'.
Add '#define SH_ETH_PM_OPS' to facilitate initialization of driver's 'pm' field
depending on whether CONFIG_PM is enabled.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
[Sergei: added the changelog, reworded the subject, changing the prefix.]
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>