TIMER1/2 should not share the same definition as TIMER0

At least on the nRF51822 and nRF51422, only TIMER0 can do 8/16/24 and 32-bit operation, while TIMER1 and 2 are restricted to 8/16-bit operation.

Because all of them share the same code, you can still do things like this:

```rust
p.device.TIMER1.bitmode.write(|w| w.bitmode()._32bit());
```

...which will put TIMER1 in either 8 or 16-bit mode, instead of the intended 32-bit mode.