net: systemport: Implement RX coalescing control knobs

Similarly to the TX path, allow the RX path to be configured with both
'rx-frames' and 'rx-usecs' coalescing parameters.

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