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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275207 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275208 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f