We don't use the hypervisor interrupt, make it optional in the device tree.

Submitted by:	Julien Grall <julien.grall AT linaro.org>
MFC after:	1 week
Some device tree configurations place the generic timer under the root
of the tree and not under simplebus. Update the driver to handle this.

Submitted by:	Julien Grall <julien.grall AT linaro.org>
MFC after:	1 week