mem: Fix address interleaving bug in DRAM controller

mem: Fix address interleaving bug in DRAM controller

This patch fixes a bug in the DRAM controller address decoding. In
cases where the DRAM burst size (e.g. 32 bytes in a rank with a single
LPDDR3 x32) was smaller than the channel interleaving size
(e.g. systems with a 64-byte cache line) one address bit effectively
got used as a channel bit when it should have been a low-order column
bit.

This patch adds a notion of "columns per stripe", and more clearly
deals with the low-order column bits and high-order column bits. The
patch also relaxes the granularity check such that it is possible to
use interleaving granularities other than the cache line size.

The patch also adds a missing M5_CLASS_VAR_USED to the tCK member as
it is only used in the debug build for now.