ext: Import Atmel SAMD20 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 SAMD20 Series Device Support (1.2.91)
License: Apache-2.0
URL: http://packs.download.atmel.com/Atmel.SAMD20_DFP.1.2.91.atpack


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

Signed-off-by: Sean Nyekjaer <sean@nyekjaer.dk>