* machine/atsamd21: implement PWM interface for all pins that support it
* machine/atsamd21: correct PWM channel mapping for pin PA18
* machine/atsamd21: move clock init into InitPWM() to hopefully save power
* machine/uart: add core support for multiple UARTs by allowing for multiple RingBuffers
* machine/uart: complete core support for multiple UARTs
* machine/uart: no need to store pointer to UART, better to treat like I2C and SPI
* machine/uart: increase ring buffer size to 128 bytes
* machine/uart: improve godocs comments and use comma-ok idiom for buffer Put/Get methods