net: systemport: add Wake-on-LAN support

Support for Wake-on-LAN using Magic Packet with or without SecureOn
password is implemented doing the following:

- setting the password to the relevant UniMAC registers
- flagging the device as a wakeup source for the system, as well as
  its Wake-on-LAN interrupt
- prepare the hardware for entering WoL mode
- enabling the MPD interrupt to wake us

The Device Tree binding documentation is also reflected to specify the
third optional Wake-on-LAN interrupt line.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>