stm32f405_timer: Update based on upstream attempt

Signed-off-by: Alistair Francis <alistair23@gmail.com>
HACK: Add comments to indicate the clock units

Signed-off-by: Alistair Francis <alistair23@gmail.com>
stm32f405_timer: Update to use correct units

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