ptp_clock: Allow for it to be optional

In order to break the hard dependency between the PTP clock subsystem and
ethernet drivers capable of being clock providers, this patch provides
simple PTP stub functions to allow linkage of those drivers into the
kernel even when the PTP subsystem is configured out. Drivers must be
ready to accept NULL from ptp_clock_register() in that case.

And to make it possible for PTP to be configured out, the select statement
in those driver's Kconfig menu entries is converted to the new "imply"
statement. This way the PTP subsystem may have Kconfig dependencies of
its own, such as POSIX_TIMERS, without having to make those ethernet
drivers unavailable if POSIX timers are cconfigured out. And when support
for POSIX timers is selected again then the default config option for PTP
clock support will automatically be adjusted accordingly.

The pch_gbe driver is a bit special as it relies on extra code in
drivers/ptp/ptp_pch.c. Therefore we let the make process descend into
drivers/ptp/ even if PTP_1588_CLOCK is unselected.

Signed-off-by: Nicolas Pitre <[email protected]>
Acked-by: Richard Cochran <[email protected]>
Acked-by: Edward Cree <[email protected]>
Acked-by: Thomas Gleixner <[email protected]>
Acked-by: John Stultz <[email protected]>
Reviewed-by: Josh Triplett <[email protected]>
Cc: Paul Bolle <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: Michal Marek <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>

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