mmu.py

Cache translations for pages rather than by exact address. This shows
a large improvement in efficiency for some specific cases:

Before:
[9/33] --dmesg ... 18.810140s
[23/33] --print-rtb ... 78.879237s
[26/33] --print-tasks ... 29.115214s

After:
[9/33] --dmesg ... 0.952276s
[23/33] --print-rtb ... 7.542078s
[26/33] --print-tasks ... 4.092283s

Change-Id: I1268928e2f1f0493b9047497a0f91b86ad83a296