ARM: DTS: stm32: add stm32f746-disco device tree files

This patch adds the DTS source files needed for stm32f746-disco board
The files are based on the stm32f429/469 files from current linux
kernel.

Source for "arch/arm/dts/armv7-m.dtsi": Linux: "arch/arm/boot/dts/armv7-m.dtsi"

Signed-off-by: Michael Kurz <michi.kurz@gmail.com>
Acked-by: Vikas MANOCHA <vikas.manocha@st.com>
diff --git a/configs/stm32f746-disco_defconfig b/configs/stm32f746-disco_defconfig
index 9e0210e..0499f1e 100644
--- a/configs/stm32f746-disco_defconfig
+++ b/configs/stm32f746-disco_defconfig
@@ -3,6 +3,7 @@
 CONFIG_STM32F7=y
 CONFIG_TARGET_STM32F746_DISCO=y
 # CONFIG_MMC is not set
+CONFIG_DEFAULT_DEVICE_TREE="stm32f746-disco"
 CONFIG_BOOTDELAY=3
 # CONFIG_DISPLAY_CPUINFO is not set
 # CONFIG_DISPLAY_BOARDINFO is not set