ks8851: suspend resume support

Add suspend/resume support using default open/stop interface methods
to do hardware dependant operations.

On suspend, same low power state (soft power mode) will be kept, the
following blocks will be disabled:

 - Internal PLL Clock
 - Tx/Rx PHY
 - MAC
 - SPI Interface

Signed-off-by: Abraham Arce <x0066660@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>