ARM: dts: STi: Add fake reg property for irq-syscfg

Add fake reg property to irq-syscfg node.
This allows to fix the following warning when compiling
dtb with W=1 option:

arch/arm/boot/dts/stih407-b2120.dtb: Warning (simple_bus_reg):
Node /soc/irq-syscfg missing or empty reg/ranges property
arch/arm/boot/dts/stih410-b2120.dtb: Warning (simple_bus_reg):
Node /soc/irq-syscfg missing or empty reg/ranges property
arch/arm/boot/dts/stih410-b2260.dtb: Warning (simple_bus_reg):
Node /soc/irq-syscfg missing or empty reg/ranges property
arch/arm/boot/dts/stih418-b2199.dtb: Warning (simple_bus_reg):
Node /soc/irq-syscfg missing or empty reg/ranges property
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>

arch/arm/boot/dts/stih407-family.dtsi

@@ -181,8 +181,9 @@ syscfg_lpm: lpm-syscfg@94b5100 {
 			reg = <0x94b5100 0x1000>;
 		};
 
-		irq-syscfg {
+		irq-syscfg@0 {
 			compatible    = "st,stih407-irq-syscfg";
+			reg = <0 0>;
 			st,syscfg     = <&syscfg_core>;
 			st,irq-device = <ST_IRQ_SYSCFG_PMU_0>,
 					<ST_IRQ_SYSCFG_PMU_1>;