Add FIFO watermark functionality.
Functionality shall be sth like this:
- FIFO Watermark level configurable
When FIFO word count is above the watermark, watermark status is asserted. If it drops below, it is de-asserted. Interrupt from Watermark shall be derived. We shall think if this should be level-based, or interrupt is captured only when water-mark is reached.
This might be a handy feature for future DMA implementation.