Correct the ilog2() for calculating memory sizes.

TLB1 can handle ranges up to 4GB (through e5500, larger in e6500), but
ilog2() took a unsigned int, which maxes out at 4GB-1, but truncates
silently. Increase the input range to the largest supported, at least for
64-bit targets. This lets the DMAP be completely mapped, instead of only
1GB blocks with it assuming being fully mapped.