modules: nrf_wifi: spi: Implement frequency switching

In order to support higher frequencies than 8MHz, we need to implement
frequency switching while waking up the RPU, as it has to be done only
at 8MHz, once RPU is awake, we can switch back to the configured (DTS)
frequency.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>