spi/imx: make spi_imx_data.devtype_data member point to const data

This prepares *of_device_id.data becoming const. Without this change the
following warning would occur:

	drivers/spi/spi-imx.c: In function spi_imx_probe:
	drivers/spi/spi-imx.c:814:24: warning: assignment discards const qualifier from pointer target type [enabled by default]
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

drivers/spi/spi-imx.c

@@ -97,7 +97,7 @@ struct spi_imx_data {
 	const void *tx_buf;
 	unsigned int txfifo; /* number of words pushed in tx FIFO */
 
-	struct spi_imx_devtype_data *devtype_data;
+	const struct spi_imx_devtype_data *devtype_data;
 	int chipselect[0];
 };