mx6: Select MX6QDL option via Kconfig

Currently the MX6QDL option is selected via CONFIG_SYS_EXTRA_OPTIONS,
but it is better to select it directly via Kconfig.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
diff --git a/configs/cm_fx6_defconfig b/configs/cm_fx6_defconfig
index 68ae70e..0773aca 100644
--- a/configs/cm_fx6_defconfig
+++ b/configs/cm_fx6_defconfig
@@ -13,7 +13,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="imx6q-cm-fx6"
 CONFIG_AHCI=y
 CONFIG_OF_BOARD_SETUP=y
-CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/mach-imx/spl_sd.cfg,MX6QDL"
+CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/mach-imx/spl_sd.cfg"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR=0x80