pseries: Add a phandle to the xicp interrupt controller device tree node

pseries: Add a phandle to the xicp interrupt controller device tree node

Future devices we will be adding to the pseries machine (e.g. PCI) will
need nodes in the device tree which explicitly reference the top-level
interrupt controller via interrupt-parent or interrupt-map properties.

In order to do this, the interrupt controller node needs an assigned
phandle.  This patch adds the appropriate property, in preparation.

Signed-off-by: David Gibson <[email protected]>
Signed-off-by: Alexander Graf <[email protected]>