net: systemport: Implement TX coalescing control knobs

Add the ability to configure both 'tx-frames' which controls how many frames
are doing to trigger a single interrupt and 'tx-usecs' which dictates how long
to wait before an interrupt should be services.

Since our timer resolution is close to 8.192 us, we round up to the nearest
value the 'tx-usecs' timeout value.

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