sfxge: add common code support for changing TX queue pace

sfxge: add common code support for changing TX queue pace

To delay packets from a particular TX queue by a particular time, write a value
into the TX Pace table s.t. pace time <= TX Pace Clock Period * (2 ^ pace value)
- the TX pace clock is 1/13 of the system clock, so its period should be 104 or
52 ns depending on whether turbo mode is active.

EFX_TX_PACE_CLOCK_BASE added by me.

Submitted by:   Mark Spender <mspender at solarflare.com>
Sponsored by:   Solarflare Communications, Inc.
Approved by:    gnn (mentor)