ext: Integrate Atmel SDK (ASF) files into Zephyr

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

This patch adds build infrastructure files like Makefile
and Kconfig to introduce ASF into Zephyr.

Jira: ZEP-976
Change-Id: I52eead376f6c1a057d6f03292ef77ff00f1b8708
Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>