ARM: dts: stm32mp1: add ldtc pre-reloc proper in SOC file

The pre-relocation probe is needed to reserve video frame buffer
in video_reserve() for all the board;
LDTC must be tagged prereloc in SOC U-Boot dtsi file.
Signed-off-by: NPatrick Delaunay <patrick.delaunay@st.com>

arch/arm/dts/stm32mp157-u-boot.dtsi

@@ -115,6 +115,11 @@
 	u-boot,dm-pre-reloc;
 };
 
+/* pre-reloc probe = reserve video frame buffer in video_reserve() */
+&ltdc {
+	u-boot,dm-pre-proper;
+};
+
 &pinctrl {
 	u-boot,dm-pre-reloc;
 };