net: add driver hook for tx time stamping.

This patch adds a hook for transmit time stamps. The transmit hook
allows a software fallback for transmit time stamps, for MACs
lacking time stamping hardware. Using the hook will still require
adding an inline function call to each MAC driver.

Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@davemloft.net>