Unable to map different alt functions on the same pad to a TimerPin

[boondocklabs]

I have added some additional pin mappings for timer outputs in #2, however there was an issue mapping them all, as it results in a conflicting trait implementation for a pad that has multiple alternate functions mapping to different channels of the timer.

For example on the STM32C011, PB6 can be either TIM1 Channel2 on AF11 or Channel3 on AF1, so we can't do:

timer_pins!(TIM1, [
    (Channel2, PB6<DefaultMode>, AltFunction::AF11),
    (Channel3, PB6<DefaultMode>, AltFunction::AF1),
]);

which results in

error[E0119]: conflicting implementations of trait `TimerPin<stm32c0::stm32c031::TIM1>` for type `gpio::gpiob::PB6<gpio::Analog>`
   --> src\timer\pins.rs:33:13
    |
33  | /             impl TimerPin<$TIMX> for $pin {
34  | |                 type Channel = $ch;
35  | |
36  | |                 fn setup(&self) {
...   |
42  | |                 }
43  | |             }
    | |             ^
    | |             |
    | |_____________first implementation here
    |               conflicting implementation for `gpio::gpiob::PB6<gpio::Analog>`