ppc/pnv: add initial IPMI sensors for the BMC simulator

Skiboot, the firmware for the PowerNV platform, expects the BMC to
provide some specific IPMI sensors. These sensors are exposed in the
device tree and their values are updated by the firmware at boot time.

Sensors of interest are :

	"FW Boot Progress"
	"Boot Count"

As such a device is defined on the command line, we can only detect
its presence at reset time.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>