STM32F405: Fix a typo in the previous commits

Also starting to split off the SYSCFG device

Signed-off-by: Alistair Francis <alistair23@gmail.com>
STM32F405_SYSCFG: Split up the SYSCFG device

Also let the EXTI device call the SYSCFG. Interrupts might now be
working, just need a way to call them

Signed-off-by: Alistair Francis <alistair23@gmail.com>