ARM: dts: imx6ull-colibri: update device trees to support overlays

Prepare in-tree device trees for out-of-tree device tree overlay support
(eMMC SKU only).

Relocate panel-dpi default to edt,et057090dhu (RGB 18bit VGA 640x480)
to the module-level dtsi and remove it from the carrier board dtsi.

Keep backlight, resistive touch and Atmel maxtouch nodes enabled
for both eMMC and NAND modules.

Signed-off-by: Denys Drozdov <denys.drozdov@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>