rpmsg: core: Make rpmsg_class constant

Since commit 43a7206b ("driver core: class: make class_register() take
a const *"), the driver core allows for struct class to be in read-only
memory, so move the rpmsg_class structure to be declared at build time
placing it into read-only memory, instead of having to be dynamically
allocated at boot time.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Link: https://lore.kernel.org/r/20240305-class_cleanup-remoteproc-v1-1-19373374e003@marliere.net


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