iomux-mx25: configuration to support CSPI3 on CSI pins

This adds necessary defines for the CSPI3 module (activated with
imx25_add_spi_imx2()) to work on the CSI pins (alternative mode 7).

Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>