us ticker API - get_us_ticker_data, us_ticker_irq_handler - declaration should live elsewhere

These 2 functions are implemented in mbed-drivers, and are generic for any us ticker. We might consider adding notes to the header file or move these declarations elsewhere

```
/** Get ticker's data
*
* @return The low power ticker data
*/
const ticker_data_t* get_us_ticker_data(void);

/** The wrapper for ticker_irq_handler, to pass us ticker's data
*
*/
void us_ticker_irq_handler(void);
```

File reference: https://github.com/ARMmbed/mbed-hal/blob/master/mbed-hal/us_ticker_api.h#L35

Start with mbed-hal/us_ticker_api.h around line 35 and compare the declarations of get_us_ticker_data and us_ticker_irq_handler with their implementations in mbed-drivers. Determine whether the declarations should be moved or documented in the header, then verify that the resulting HAL and driver interfaces remain consistent.

Written by the indexing model from the issue text.