spi_imx: Fix misspelled configuration variable SOC_IMX31

This fix a kernel NULL pointer error while initialising SPI introduced
by: 4d2f13be1e370a670c1cae20c194d5ce961e0fa5

Signed-off-by: Alberto Panizzo <alberto@amarulasolutions.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

drivers/spi/Kconfig

@@ -161,7 +161,7 @@ config SPI_IMX_VER_0_0
 	def_bool y if SOC_IMX21 || SOC_IMX27
 
 config SPI_IMX_VER_0_4
-	def_bool y if SOC_MX31
+	def_bool y if SOC_IMX31
 
 config SPI_IMX_VER_0_7
 	def_bool y if ARCH_MX25 || SOC_IMX35 || SOC_IMX51 || SOC_IMX53