ext: Import Atmel SAM3X 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 SAM3X Series Device Support (1.0.50)
URL: http://packs.download.atmel.com/Atmel.SAM3X_DFP.1.0.50.atpack


License: Apache-2.0
Maintained-by: External

Jira: ZEP-976
Change-Id: Icff0e6d899ea1bd9fb12cb03ca0c3644cac687d9
Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>