net: fec: optimize the clock management to save power

Add below clock management to save fec power:
- After probe, disable all clocks incluing ipg, ahb, enet_out, ptp clock.
- Open ethx interface enable necessary clocks.
  Close ethx interface disable all clocks.

The patch also encapsulates the all enet clocks enable/disable to
.fec_enet_clk_enable(), which can reduce the repetitional code in
driver.

Signed-off-by: Fugang Duan <B38611@freescale.com>
Acked-by: Frank Li <Frank.li@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>