ARM: dts: Add stdout-path property to i.MX boards

This adds the stdout-path property to various i.MX boards. Values
of the property have been taken from barebox, so they should be
correct. Also, the older linux,stdout-path property is converted
to stdout-path.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>