MIPS: generic: enable SMP on SMVP systems

In addition to CPS SMP setups, also try to initialise MT SMP setups with
multiple VPEs per CPU core. CMP SMP support is not provided as it is
considered deprecated.

Additionally, rework the code by dropping the err variable and make it
similar to how other platforms perform this initialisation.

Co-developed-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Signed-off-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>