50.6.2 SCAN Control register

Bug found by Chronologist, https://forum.pjrc.com/threads/29624-Using-TSI-for-capacitance-measurement

https://forum.pjrc.com/threads/29446-Teensy-Hardware-Flow-Control-RTS-CTS?p=80425#post80425

can now receive and send Midi Time Codes
and send Real Time messages

https://forum.pjrc.com/threads/28932-LC-is-10-9-times-slower-than-T3-1?p=76072&viewfull=1#post76072

https://forum.pjrc.com/threads/25153-PDB0CH1-register-definitions

https://forum.pjrc.com/threads/28932-LC-is-10-9-times-slower-than-T3-1?p=75775&viewfull=1#post75775

https://forum.pjrc.com/threads/28932-LC-is-10-9-times-slower-than-T3-1

https://forum.pjrc.com/threads/28325-Teensy-LC-and-delay

or, alternativly one could write size_t strlen(const char *s) {return __builtin_strlen(s);} to use the gcc-builtin function.
I think, both are faster than the bytewise search.

systick_millis_count is already volatile, doing it twice is not needed.

- Added bit definitions for the PIT
- limited Teensy LC to only two PITs
- added PIT_LTMR64 for Teensy LC