ARM: dts: stm32mp: Add iwdg2 support for stm32mp157c

This patch adds independent watchdog support for stm32mp157c
in SPL.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

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

@@ -140,3 +140,7 @@
 	compatible = "st,stm32-gpio";
 	u-boot,dm-pre-reloc;
 };
+
+&iwdg2 {
+	u-boot,dm-pre-reloc;
+};