ARM: dts: imx6qdl-dhcom-pdk2: Fill in missing panel power-supply

ARM: dts: imx6qdl-dhcom-pdk2: Fill in missing panel power-supply

Add missing panel power-supply property to DT to fix the following warning.
The power supply on this device is very simple chain of regulators from the
main 24V input, describe those.

"
arch/arm/boot/dts/nxp/imx/imx6q-dhcom-pdk2.dtb: panel: 'power-supply' is a required property
"

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>