sfc: Add support for new board sensors

Add support for power and current sensors, which need to be named
differently in sysfs.  Power sensors also require values to be scaled
between MCDI and sysfs, and have no minimum value.

Add definitions of the power, current, fan, and additional temperature
and voltage sensors found on SFA6902F, SFN7022F and SFN7122F.

(Includes a bug fix from Andrew Jackson.)

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