ARM: tegra: paz00: enable WiFi rfkill when booting from device tree

There currently aren't bindings for a WiFi rfkill button, and defining
a good binding is non-trivial. Manually register this "device" when
booting from device tree, in order to bring DT support to the same
feature level as board files, which will in turn allow board files to be
deprecated.

Signed-off-by: Stephen Warren <swarren@nvidia.com>