netlink: specs: Introduce new netlink command to list available time stamping layers

Add a new commands allowing to list available time stamping layers on a
netdevice's link.

Example usage :
./ynl/cli.py --spec netlink/specs/ethtool.yaml --no-schema \
	     --do ts-list-get \
	     --json '{"header":{"dev-name":"eth0"}}'
{'header': {'dev-index': 3, 'dev-name': 'eth0'},
 'ts-list-layer': b'\x01\x00\x00\x00\x05\x00\x00\x00'}

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