sfc: Enable PTP clock and timestamping for all functions on EF10

The SFC9100 family has only one clock per controller, shared by all
functions.  Therefore only create a clock device under the primary
function, and make all other functions refer to the primary's clock
device.

Since PTP functionality is limited to port 0 and PF 0 on the earlier
SFN[56]322F boards, and we also set the primary flag for that
function, we can make the creation of a clock device conditional only
on this flag.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>