Add clock group

The clock group is needed to "to enable applications like linuxptp to open clocks without root privileges". See the link below: https://github.com/systemd/systemd/commit/af96ccfc24bc4803078a46b4ef2cdeb5decdfbcd

Note that this group is needed as long as people use udev from systemd (258+) because 50-udev-default.rules needs it. See the above commit.

Signed-off-by: Chen Qi Qi.Chen@windriver.com