[rtl] add generic cache module (not used yet)

[stnolting]

This PR adds a new generic cache module that will might replace the I- and D-cache modules. Right now, this new cache module is not used at all by the default processor setup.

Cache specs:

• direct-mapped, configurable block size ("cache lines"), configurable number of blocks
• write-back strategy: write back dirty cache blocks when the block is replaced
• write-allocate: a write miss will load the entire block from memory
• supports direct (uncached) memory accesses