powerpc64/powernv: Add opal NVRAM driver for PowerNV systems

Add a very basic NVRAM driver for OPAL which can be used by the IBM
powerpc-utils nvram utility, not to be confused with the base nvram utility,
which only operates on powermac_nvram.

The IBM utility handles all partitions itself, treating the nvram device as
a plain store.

An alternative would be to manage partitions in the kernel, and augment the
base nvram utility to deal with different backing stores, but that
complicates the driver significantly. Instead, present the same interface
IBM's utlity expects, and we get the usage for free.

Tested by: bdragon