powerpc/booke: Clamp MAXCPU to 32 for Book-E platforms

Since the likelihood of new Book-E PowerPC SoCs being produced is near
zero clamp MAXCPU to around the highest number of cores/threads
available currently, for both 64-bit and 32-bit. Even though the
current highest core/thread count is 24, the cap is set at 32 in case
there is code that assumes power of 2.