mem-cache: Fix non-bijective function in Skewed caches

mem-cache: Fix non-bijective function in Skewed caches

The hash() function must be bijective for the skewed caches to work,
however when the hashing is done on top of a one-bit address, the
MSB and LSB refer to the same bit, and therefore their xor will
always be zero.

This patch adds a fatal error to not allow the user to set an invalid
value for the number of sets that would generate that bug.

As a side note, the missing header for the bitfields functions has
been added.

Change-Id: I35a03ac5fdc4debb091f7f2db5db33568d0b0021
Reviewed-on: https://gem5-review.googlesource.com/12724
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>