mips: txx9_sram - convert sysdev_class to a regular subsystem

After all sysdev classes are ported to regular driver core entities, the
sysdev implementation will be entirely removed from the kernel.

Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Cc: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>