nfp: add phys_switch_id support

Add phys_switch_id support by allowing lookup of
SWITCHDEV_ATTR_ID_PORT_PARENT_ID via the nfp_repr_port_attr_get
switchdev operation.

This is visible to user-space in the phys_switch_id attribute
of a netdev.

e.g.

cd /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0
find . -name phys_switch_id | xargs grep .
./net/eth3/phys_switch_id:00154d1300bd
./net/eth4/phys_switch_id:00154d1300bd
./net/eth2/phys_switch_id:00154d1300bd
grep: ./net/eth5/phys_switch_id: Operation not supported

In the above eth2 and eth3 and representor netdevs for the first and second
physical port. eth4 is the representor for the PF. And eth5 is the PF netdev.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>