Add watermark interrupt strobe functionality

[ajn96]

Add enable bit to make watermark interrupt behave like a "data ready" signal when the interrupt is triggered.

When the bit is set, instead of keeping watermark interrupt signal constantly high, the signal will toggle at ~10KHz. This will allow a master device (specifically the EVAL-ADIS-FX3) to trigger reads on data ready, ensuring that the buffer outputs are only read/logged when buffered data is available. When the watermark interrupt is not triggered the selected line will return to 0.

[ajn96]

Done.