ptp: Fix documentation to match code.

Ever since commit 3a06c7ac ("posix-clocks: Remove interval timer
facility and mmap/fasync callbacks") the possibility of PHC based
posix timers has been removed.  In addition it will probably never
make sense to implement this functionality.

This patch removes the misleading text which seems to suggest that
posix timers for PHC devices will ever be a thing.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>