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/gwventana_nand_defconfig b/configs/gwventana_nand_defconfig
index c9e8b8b..dd4d73c 100644
--- a/configs/gwventana_nand_defconfig
+++ b/configs/gwventana_nand_defconfig
@@ -15,7 +15,7 @@
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=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_SYS_STDIO_DEREGISTER is not set
 # CONFIG_DISPLAY_BOARDINFO is not set