ARM: dts: stm32: add STM32F746 syscfg clock

The syscfg syscon was missing its clock, therefore any attempt to
read/write it after clk_disable_unused() silently failed. This was
preventing external pin interrupts from working if they were initialized
after this point.
Signed-off-by: Ben Wolsieffer <ben.wolsieffer@hefring.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>

arch/arm/boot/dts/st/stm32f746.dtsi

@@ -494,6 +494,7 @@ sdio1: mmc@40012c00 {
 		syscfg: syscon@40013800 {
 			compatible = "st,stm32-syscfg", "syscon";
 			reg = <0x40013800 0x400>;
+			clocks = <&rcc 0 STM32F7_APB2_CLOCK(SYSCFG)>;
 		};
 
 		exti: interrupt-controller@40013c00 {