Two critical issues are fixed with this commit:
1. The MMU cache was being corrupted by writing the 'Last Used'
bit into the wrong half of the PD cache (low word vs. high word)
2. The MMU cache was being too aggressively flushed on SRAMA
write, because the wrong length was being used. In addition,
the code was walking off the end of the cache array when
flushing any section other than section 0, potentially causing
memory corruption.
