powerpc/powernv: hwmon driver for power values, fan rpm and temperature

This patch adds basic kernel enablement for reading power values, fan
speed rpm and temperature values on powernv platforms which will
be exported to user space through sysfs interface.

Signed-off-by: Shivaprasad G Bhat <sbhat@linux.vnet.ibm.com>
Signed-off-by: Neelesh Gupta <neelegup@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>