mips: sibyte: make tb_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 tb_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>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>