ARM: dts: stm32: Add backlight and panel supply on DHCOM SoM

Fix the following dtbs_check warning:
arch/arm/boot/dts/stm32mp157c-dhcom-pdk2.dt.yaml: display-bl: 'power-supply' is a required property
arch/arm/boot/dts/stm32mp157c-dhcom-pdk2.dt.yaml: panel: 'power-supply' is a required property

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Cc: kernel@dh-electronics.com
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-arm-kernel@lists.infradead.org
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>