add support for Atmel SAM4L NOR Flash

add support for Atmel SAM4L NOR Flash

This adds a new NOR Flash driver, "at91sam4l", which supports the
built-in Flash on Atmel's low-power SAM4L family of Cortex M4 MCUs.
Parts and their geometry are detected automatically using the Chip ID
and lookup schemes described in document 42023E–SAM–07/2013.

Tested on AT91SAM4LC4CA via the SAM4L XPlained Pro development kit.

Change-Id: If73499dee92cc8ce231845244ea25c6984f6cecd
Signed-off-by: Andrey Yurovsky <yurovsky@gmail.com>
Reviewed-on: http://openocd.zylin.com/1639
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>