disas: QOMify mips specific disas setup

Move the target_disas() mips specifics to the CPUClass::disas_set_info()
hook and delete the #ifdef specific code in disas.c.

Cc: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Acked-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>