per-chip atsamd-HAL docs

ChipFeatures
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