stm32f405_timer: Update to a ns timer

Signed-off-by: Alistair Francis <alistair23@gmail.com>
stm32f405_syscfg: Update based on upstream attempt

Signed-off-by: Alistair Francis <alistair23@gmail.com>
stm32f405_timer: Update based on upstream attempt

Signed-off-by: Alistair Francis <alistair23@gmail.com>
stm32f405_usart: Update based on upstream attempt

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