mx51_3ds: Add support for the imx51 3-stack board

3-stack is a reference board from Freescale for their i.MX51 SoC.

Add board definition, Kconfig and Makefile to enable Freescale 3-stack
board.

Boot tested on a i.MX51 3-stack Rev2.0 board

Signed-off-by: Jason Wang <jason77.wang@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>