powerpc/powernv: Add OPAL check token call

Currently there is no way to generically check if an OPAL call exists or not
from the host kernel.

This adds an OPAL call opal_check_token() which tells you if the given token is
present in OPAL or not.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>