netlink: specs: Introduce new netlink command to get current timestamp

Add a new commands allowing to get the current time stamping on a
netdevice's link.

Example usage :
./ynl/cli.py --spec netlink/specs/ethtool.yaml --no-schema --do ts-get \
	     --json '{"header":{"dev-name":"eth0"}}'
{'header': {'dev-index': 3, 'dev-name': 'eth0'}, 'ts-layer': 1}

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>