Add FreqMeasure driver for Teensy 4.1

[PaulStoffregen]

Can support 4 instances. Tuner applet still uses only 1.

[PaulStoffregen]

Opps, I left a few Serial.print() in the code. Please delete or comment them out.

[djphazer]

This has been merged in for v1.8 - 1dd4b17

Along with some follow-up commits:
208ae72
3b43898

T4.0 works fine for TR1/TR2 (pin 0/1) but not for TR3/TR4 (pin 2/3)
It will work for pin 23/22 but those are connected to the encoders on
original hardware. I imagine the driver code needs different parameters
for FlexPWM to work on pins 2/3 instead (or maybe QuadTimer will also
work on pins 2/3), but I can't swim through the data sheets to figure it out!

Basically, Teensy 4.0 might need more attention if we want TR3/TR4 to work, but this is more than satisfactory!