powerpc/powernv: Enable fetching of platform sensor data

This patch enables fetching of various platform sensor data through
OPAL and expects a sensor handle from the driver to pass to OPAL.

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