mx6slevk: Convert to OF_UPSTREAM

Instead of using the local imx6sl-evk, imx6sl-pinfunc.h and
imx6sl.dtsi devicetree copies from U-Boot, convert the imx6sl-evk board
to OF_UPSTREAM so that the upstream kernel devicetree can be used instead.

Signed-off-by: Fabio Estevam <festevam@denx.de>
diff --git a/configs/mx6slevk_defconfig b/configs/mx6slevk_defconfig
index 071bcae..e927bb2 100644
--- a/configs/mx6slevk_defconfig
+++ b/configs/mx6slevk_defconfig
@@ -9,7 +9,7 @@
 CONFIG_MX6SL=y
 CONFIG_TARGET_MX6SLEVK=y
 CONFIG_DM_GPIO=y
-CONFIG_DEFAULT_DEVICE_TREE="imx6sl-evk"
+CONFIG_DEFAULT_DEVICE_TREE="nxp/imx/imx6sl-evk"
 # CONFIG_CMD_BMODE is not set
 CONFIG_SUPPORT_RAW_INITRD=y
 CONFIG_USE_BOOTCOMMAND=y