tests: boards: nrf52_bsim: egu: Replace obsolete timer frequency API

New nrfx release removes depricated `nrf_timer_frequency_set()`
function. Instead of it `nrf_timer_prescaler_set` is used with
`NRF_TIMER_PRESCALER_CALCULATE()` and `NRF_TIMER_BASE_FREQUENCY_GET()`
macros.

Signed-off-by: Adam Wojasinski <adam.wojasinski@nordicsemi.no>