powernv: opal-sensor: Add support to read 64bit sensor values

This patch adds support to read 64-bit sensor values. This method is
used to read energy sensors and counters which are of type u64.

Signed-off-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>