ext: Import Atmel SAM4S 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 SAM4S Series Device Support (1.0.56)
URL: http://packs.download.atmel.com/Atmel.SAM4S_DFP.1.0.56.atpack


Maintained-by: External

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>