ARM: dts: stm32: convert stm32mp15 board to OF_UPSTREAM

Enable OF_UPSTREAM flag for STM32MP15 platforms, except for
stm32mp15-odyssey,see following patch :

"configs: stm32: introduce stm32mp15-odyssey_defconfig"

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/arch/arm/mach-stm32mp/Kconfig b/arch/arm/mach-stm32mp/Kconfig
index 3a5f488..7ba928c 100644
--- a/arch/arm/mach-stm32mp/Kconfig
+++ b/arch/arm/mach-stm32mp/Kconfig
@@ -74,6 +74,7 @@
 	select SUPPORT_SPL
 	select SYS_ARCH_TIMER
 	imply CMD_NVEDIT_INFO
+	imply OF_UPSTREAM
 	help
 		support of STMicroelectronics SOC STM32MP15x family
 		STM32MP157, STM32MP153 or STM32MP151