Update gpiote.rs

Adding these changes enables us to define a channel using a mutable reference to `GPIOTE_CH(n)`, similar to how we can do with other drivers.
So instead of using:
```rust
let freq_in = InputChannel::new(
    p.GPIOTE_CH0,
    Input::new(&mut p.P0_19, embassy_nrf::gpio::Pull::Up),
    embassy_nrf::gpiote::InputChannelPolarity::HiToLo,
);
```
we can use:
```rust
let freq_in = InputChannel::new(
    &mut p.GPIOTE_CH0,
    Input::new(&mut p.P0_19, embassy_nrf::gpio::Pull::Up),
    embassy_nrf::gpiote::InputChannelPolarity::HiToLo,
);
```

embassy-nrf/src/gpiote.rs

@@ -419,6 +419,12 @@ macro_rules! impl_channel {
                 $number as usize
             }
         }
+        impl sealed::Channel for &mut peripherals::$type {}
+        impl Channel for &mut peripherals::$type {
+            fn number(&self) -> usize {
+                $number as usize
+            }
+        }
     };
 }