net_tstamp: Add TIMESTAMPING SOFTWARE and HARDWARE mask

Timestamping software or hardware flags are often used as a group,
therefore adding these masks will easier future use.

I did not use SOF_TIMESTAMPING_SYS_HARDWARE flag as it is deprecated and
not use at all.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>