🐛 Make sure IMEM/DMEM sizes are a power of two

[stnolting]

The address logic of the internal memories as well as the central address decoding of the bus system cannot cope with address space sizes, which are not a power of two. This is especially true for the internal IMEM and DMEM memories. This PR fixes this problem by ensuring that the configured memory sizes are always a power of two:

If the configured memory size (via the MEM_INT_IMEM_SIZE / MEM_INT_DMEM_SIZE generic) is not a power of two the actual memory size will be auto-adjusted to the next power of two (e.g. configuring a memory size of 60kB will result in a physical memory size of 64kB).

👍 Identified by GitHub user @cmenendezzz in #642.

🐛 This is not a real bug, but something that can cause problems.