board/freescale/imx6ulevk/readme.txt: update broken url

http://freescale.com/ urls are all redirected to the NXP homepage
https://www.nxp.com/

Even if the link is not broken in the sense of a 404 http error, the
pointed resource is not found.

This commit updates the old freescale link to the correct nxp.com
location.

Note: the link now requires a free registration to be accessed.

Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

board/freescale/imx6ulevk/readme.txt

@@ -71,4 +71,4 @@ Enjoy!
 
 References
 ==========
-[1] http://cache.freescale.com/files/32bit/doc/quick_start_guide/IMX6ULTRALITEQSG.pdf
+[1] https://www.nxp.com/webapp/Download?colCode=IMX6ULTRALITEQSG