net: systemport: add suspend and resume support

Implement the hardware recommended suspend/resume procedure for
SYSTEMPORT. We leverage the previous factoring work such that we can
logically break all suspend/resume operations into disctint RX and TX
code paths.

When the system enters S3, we will loose all register contents, so
make sure that we correctly re-program all the hardware and software
views of the RX & TX rings as well.

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