Changes to SAMD related files:

• refactoring of the code, to extract SAMD21 and SAMD51 commonalities to their own file, add SAMD51 specific file, and remove no longer needed files
• make the pin-permutation code support different numbers of peripherals per pin (SAMD21 has 2 SAMD51 has 3)
• support SAMD51 (initial support, 3-PWM working, TC not yet working, PWM is still fixed frequency)
• set duty cycle via buffer registers - guarantees glitch-free PWM changes (SAMD51 & SAMD21)