rpmsg: core: Make rpmsg_bus const

Now that the driver core can properly handle constant struct bus_type,
move the rpmsg_bus variable to be a constant structure as well,
placing it into read-only memory which can not be modified at runtime.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20240204-bus_cleanup-rpmsg-v1-1-1703508c23b7@marliere.net


Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>