ARM: mx5/iomux-mx51: add iomux definitions for eCSPI2 on the imx51_3ds board

On the imx51_3ds board, eCSPI2 is connected to a SPI NOR flash,
now add iomux definitions for those used pins.

Signed-off-by: Jason Wang <jason77.wang@gmail.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>