nfp: add hwmon support

Add support for retrieving temperature and power sensor and limits via NSP.

Signed-off-by: David Brunecz <david.brunecz@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>