sfc: Add support for SFC9100 timestamp format

The clock minor tick on the SFC9100 family is 2^-27 s, not 1 ns.
There are also various pipeline delays which we need to correct for
when interpreting timestamps.

We query the firmware for the clock format and corrections at run-time.

[bwh: Combined and rebased several changes]
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>