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>