powerpc/powernv: Expose OPAL APIs required by PRD interface

The (upcoming) opal-prd driver needs to access the message notifier and
xscom code, so add EXPORT_SYMBOL_GPL macros for these.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>