powerpc/powernv: Expose OPAL firmware symbol map

Newer versions of OPAL will provide this, so let's expose it to user
space so tools like perf can use it to properly decode samples in
firmware space.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>