MIPS: generic: Allow R5 CPUs to be selected

Since we do have P5600 bitfile for boston board, we should
allow generic kernel to be compiled for R5 CPUs.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>