mx31_3ds: Fix spi registration

Commit (06606ff1) "ARM: imx: dynamically register spi_imx devices
(imx31)" registered the wrong SPI port for mx31_3ds board.
mx31_3ds uses SPI2 to connect to the MC13783 PMIC.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Rogerio Pimentel <rogerio.pimentel@freescale.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>