pps, ptp: Remove dependencies on EXPERIMENTAL

These are now established subsystems, and we want drivers to be able
to select PPS and PTP_1588_CLOCK without depending on EXPERIMENTAL.
Further, the use of EXPERIMENTAL is now deprecated in general.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>