Remove PPS GPIO interrupt when the rate is higher than 20Hz

If the PPS GPIO is exposed to a signal with high frequency changes, a lot of
interrupts are scheduled and the handling of these calls can worst-case
starve flight critical processes leading to a loss of
control. Since PPS is not flight critical, we now give up the PPS
functionality and stop the interrupts to prevent the starvation of other processes.

msg/pps_capture.msg

@@ -1,2 +1,3 @@
 uint64 timestamp			  # time since system start (microseconds) at PPS capture event
-uint64 rtc_timestamp		# Corrected GPS UTC timestamp at PPS capture event
+uint64 rtc_timestamp		# Corrected GPS UTC timestamp at PPS capture event
+uint8  pps_rate_exceeded_counter # Increments when PPS dt < 50ms