package: linuxptp: enhance systemd service

linuxptp now pulls in the time-sync.target, which other services can
use to make sure they have a valid time before starting.

phc2sys (linuxptp-system-clock.service) starts after ptp4l
(linuxptp.service) so we have a valid time before writing it to the
hardware clock.

Signed-off-by: Petr Kulhavy <brain@jikos.cz>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>