ppc46 iSeries: Make some generic irq code compile for iSeries

In order to call finish_device_tree() on iSeries we need to define
virt_irq_create_mapping(). We also need to set ppc64_interrupt_controller to
something other than zero. If we want to do interrupt setup via the device
tree on iSeries this code will need some serious work, but it's harmless to
have it there as long as the nodes in the iSeries device tree don't cause
it to be invoked.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>