ARM: dts: stm32: fix ltdc warnings in stm32mp15 boards

Those concern:
  * "#size-cells" and "#address-cells" wrongly used
  * residual "reg" property appearing on endpoints where it could be
    avoided

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>