imx: mx6slevk: enable DM_SPI and DM_SPI_FLASH

With DM_SPI and DM_SPI_FLASH enabled, we could get:

spi          0  [   ]   mxc_spi               |   |   |   `-- ecspi@02008000
spi_flash    0  [   ]   spi_flash_std         |   |   |       `-- m25p80@0

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/configs/mx6slevk_defconfig b/configs/mx6slevk_defconfig
index e614ef2..6a7fa13 100644
--- a/configs/mx6slevk_defconfig
+++ b/configs/mx6slevk_defconfig
@@ -35,6 +35,7 @@
 CONFIG_DM_I2C=y
 CONFIG_DM_MMC=y
 CONFIG_FSL_USDHC=y
+CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH=y
 CONFIG_SF_DEFAULT_MODE=0
 CONFIG_SF_DEFAULT_SPEED=20000000
@@ -50,6 +51,7 @@
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_DM_REGULATOR_GPIO=y
 CONFIG_SPI=y
+CONFIG_DM_SPI=y
 CONFIG_MXC_SPI=y
 CONFIG_DM_THERMAL=y
 CONFIG_USB=y