ext: Import Atmel SAMR21 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 SAMR21 Series Device Support (1.1.72)
License: Apache-2.0
URL: http://packs.download.atmel.com/Atmel.SAMR21_DFP.1.1.72.atpack


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

Signed-off-by: Benjamin Valentin <benpicco@googlemail.com>