add support for Atmel SAMD NOR Flash

add support for Atmel SAMD NOR Flash

This adds a new NOR Flash driver, "at91samd", which supports the
built-in Flash on Atmel's D-series Cortex M MCUs, starting with the D20.
Parts and their geometry are detected automatically using the DSU and
lookup schemes described in the D20 document, 42129F–SAM–10/2013.
Future D-series variants and families should presumably use this
controller as well (possibly with minor changes and improvements).

Tested on the SAMD20 Xplained Pro board, for which we also add the
corresponding Flash configuration.

Change-Id: Id8d3dd601e9f53121682d1a1190d0be4ea3b83eb
Signed-off-by: Andrey Yurovsky <yurovsky@gmail.com>
Reviewed-on: http://openocd.zylin.com/1684
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>