Add compat strings for all the flavors of GIC this driver should support.
Also allow the driver to attach to ofwbus as well as simplebus, some FDT
data puts the root interrupt controller on the root bus.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271595 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f