ARM: dts: stm32: fix dsi warnings on stm32mp15 boards

Fixes DSI related warnings:

 * "#size-cells" and "#address-cells" wrongly used
 * Changed 'panel-dsi@0' to 'panel@0' according to dsi-controller.yaml

Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>