xive: add a get_tctx() method to the XiveRouter

xive: add a get_tctx() method to the XiveRouter

It provides a mean to retrieve the XiveTCTX of a CPU. This will become
necessary with future changes which move the interrupt presenter
object pointers under the PowerPCCPU machine_data.

The PowerNV machine has an extra requirement on TIMA accesses that
this new method addresses. The machine can perform indirect loads and
stores on the TIMA on behalf of another CPU. The PIR being defined in
the controller registers, we need a way to peek in the controller
model to find the PIR value.

The XiveTCTX is moved above the XiveRouter definition to avoid forward
typedef declarations.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>