Logical devices created by the host1x bus infrastructure don't need to
be associated with a device tree node. Doing so will cause the driver
core to attempt to hook up IOMMU operations and fail because it is not
a real device.

However, for backwards-compatibility, we need to provide various OF_*
uevent variables that were previously provided by of_device_uevent() and
which are parsed by libdrm in userspace when querying the available
devices. Do this by implementing a uevent callback for the host1x bus.
Signed-off-by: Thierry Reding <treding@nvidia.com>