Config option IMXRT_ADCx_ETC can now be used to select an external HW
trigger to be used instead of continous trigger. Continous trigger is
used if IMXRT_ADCx_ETC = -1 (default option). Otherwise the source signal
is routed through XBAR and used as a trigger.
Hardware triggering is currently limited to maximum of 8 channels.
HW trigger is automatically disabled if there are more than 8 channels.
The external triggering was tested with PWM signal as a source.
Signed-off-by: Michal Lenc <michallenc@seznam.cz>
9ebd7e525c