ext: Import Atmel SAMD21 header files from ASF library

Atmel Software Framework (ASF) provides a set of low-level header
files that give access to different hardware peripherals of Atmel's
ICs.

Origin: Atmel SAMD21 Series Device Support (1.2.276)
License: Apache-2.0
URL: http://packs.download.atmel.com/Atmel.SAMD21_DFP.1.2.276.atpack


Purpose: Introduction of ASF for the SAM0 series.
Maintained-by: External

Signed-off-by: Michael Hope <mlhx@google.com>