hw/ipmi: Rework sensors and fix boot count sensor
hw/ipmi: Rework sensors and fix boot count sensor

This is an attempt to make it clearer how to use the sensor set ipmi
command, and to do the correct thing to our BMC.

The boot count was using the incorrect set mask and type. It now confirms
to what the AMI BMC expects.

Signed-off-by: Joel Stanley <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@jms.id.au>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

open-power/skiboot,shenki/skiboot,mikey/skiboot,qemu/skiboot,csmart/skiboot,legoater/skiboot,qemu/skiboot,apopple/skiboot,qemu/skiboot,open-power/skiboot,shenki/skiboot,open-power/skiboot,shenki/skiboot,ddstreet/skiboot,apopple/skiboot,stewart-ibm/skiboot,csmart/skiboot,open-power/skiboot,mikey/skiboot,open-power/skiboot,legoater/skiboot,qemu/skiboot,qemu/skiboot,legoater/skiboot,ddstreet/skiboot,stewart-ibm/skiboot,ddstreet/skiboot,apopple/skiboot,csmart/skiboot,legoater/skiboot,stewart-ibm/skiboot,shenki/skiboot,shenki/skiboot,mikey/skiboot,legoater/skiboot