sfc: Use lm87 and lm90 drivers for board temperature/power monitoring

Add board monitoring to periodic work whenever link is down.
For SFE4001, report when a fault has caused the PHY to turn off.
For SFE4002, switch XFP PHY into low-power state in case of a fault.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>