Method 1: As far as I know the pulse goes from 1ms to 2ms (1.5ms is middle position). It should be possible to "analogify" or mathematically say "integrate" the PWM signal using a R-C combination on the input pin and read the voltage from this. But the voltage level will be low due the duty cycle which goes from approx. 1:20 (1ms/20ms) to 1:10 (2ms/20ms). From 5V full scale you read 0.25V to 0.5V. You may add an analog OPAMP amplify it to 5V. This method will slow down the reaction time a little bit
Method 2: Before doing the Method 1 it may be less effort to write it in Arduino and use either a PWM read library or use a interrupt to measure the pulse width. You can also read the low time to determine if the input signal is valid or not eg. to detect signal loss.
Method 3: This is the simple but ugly workaround! Add a small Servo to your signal and drive a potentiometer which ist then read by the M5 analog input.