stm32f405_timer: Add some comments for PWM

Signed-off-by: Alistair Francis <alistair23@gmail.com>
stm32f405_timer: Split into seperate header file

Also tidy up some naming issues

Signed-off-by: Alistair Francis <alistair23@gmail.com>
stm32f405_timer: Split the timer into a seperate config

Signed-off-by: Alistair Francis <alistair23@gmail.com>
stm32f405_timer: Update based on mailing list reviews

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