sfc: Add hwmon driver for boards using SFC9000-family controllers

The SFC9000-family controllers have firmware to manage all board
peripherals including temperature, heat sink continuity and voltage
sensors.  The firmware reports sensor alarms, which we log, and
will shut down the board if necessary.

Some users may want to monitor their boards more closely, so add an
hwmon driver that exposes all sensors reported by the firmware.  Move
efx_mcdi_sensor_event() into the new file so it can share the array of
sensor labels with the hwmon driver.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>