ptp: Make PTP_1588_CLOCK select rather than depend on PPS

PTP hardware clock drivers that select PTP_1588_CLOCK must currently
also select PPS.  For those drivers that don't, the user must enable
PPS, then enable PTP_1588_CLOCK, then the driver.  Simplify things for
developers and users by putting this selection in one place.

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