Hardware based timestamping using PRU IEP timer capture

[lsigrist]

Introduce hardware based timestamping of time between two ADC samples (i.e. the DT channel) based on the PRU IEP timer capture functionality.

• introduce PRU IEP timer capture configuration code for hardware based timestamping the time between two ADC samples
• remove use of PRU cycle counter for time delta measurement between two ADC samples
• use fixed channel calibration values (obsoletes use of DT channel calibration data)

:warning: Requires hardware modification to work: connect the ADC data ready signal to the edc_latch1_in input, e.g. by wiring extension header pin P8.15 or P8.16 to pin P9.19

For an alternative hardware timestamping using eCAP capture module see #67.

[lsigrist]

Close in favor of #67 that provides the same functionality but requires no RocketLogger hardware changes.