external_gpio: Update Python script to support interupts

Signed-off-by: Alistair Francis <alistair23@gmail.com>
external_acces: Update the GPIO device to work with interrupts

Signed-off-by: Alistair Francis <alistair23@gmail.com>
stm32f405_syscfg: Fix some incorect divisions

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