per-chip atsamd-HAL docs

Chip	Features
samd11c	samd11c unproven
samd11d	samd11d unproven
samd21g	samd21g unproven usb
samd21j	samd21j unproven usb
samd51g	samd51g unproven usb
samd51j	samd51j unproven usb
samd51n	samd51n unproven usb
samd51p	samd51p unproven usb