flash: driver for Atmel SAMV, SAMS, and SAME

This is a driver for the Atmel Cortex-M7 SAMV, SAMS, and SAME.
I started with the at91sam4.c driver and then restructured it
significantly to try to simplify it and limit the functionality
to just a flash driver, as well as to comply with the style guide.

Change-Id: I5340bf61f067265b8ebabd3adad45be45324b707
Signed-off-by: Morgan Quigley <morgan@osrfoundation.org>
Reviewed-on: http://openocd.zylin.com/2952


Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>