pseries: Return the token when we register an RTAS call

pseries: Return the token when we register an RTAS call

The kernel will soon be able to service some RTAS calls. However the
choice of tokens will still be up to userspace. To support this have
spapr_rtas_register() return the token that is allocated for an
RTAS call, that allows the calling code to tell the kernel what the
token value is.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>