Hardware based timestamping using PRU eCAP capture

[lsigrist]

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

• add PRU eCAP0 capture configuration code for hardware based timestamping the time between two ADC samples
• fix naming of GPIO inputs DR0/DR1 to nDR0/nDR1 in devicetree to reflect their active low signal semantic
• change PRU to wait for new ADC data from nDR0 to nDR1 signal
• remap nDR0 signal for use as capture input with eCAP0 module
• use fixed channel calibration values (obsoletes use of DT channel calibration data)
• remove use of PRU cycle counter for time delta measurement between two ADC samples