xen/device-tree: Add ability to handle nodes with interrupts-extended prop

xen/device-tree: Add ability to handle nodes with interrupts-extended prop

The "interrupts-extended" property is a special form for use when
a node needs to reference multiple interrupt parents.

According to:
Linux/Documentation/devicetree/bindings/interrupt-controller/interrupts.txt

But, there are cases when "interrupts-extended" property is used for
"outside /soc node" with a single interrupt parent as an equivalent of
pairs ("interrupt-parent" + "interrupts").

A good example here is ARCH timer node for R-Car Gen3/Gen2 family,
which is mandatory device for Xen usage on ARM. And without ability
to handle such nodes, Xen fails to operate.

So, this patch adds required support for Xen to be able to handle
nodes with that property.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>