drivers: spi_nrfx_spim: bring back get_nrf_spim_frequency

Commit 246393e8
("drivers: spi: spi_nrfx_spim: Remove nrf_frequency_t handling")'
introduced two changes, one of them is removing the function
get_nrf_spim_frequency with a strange justification.
This change  breaks support for peripherals written in a common way,
where the maximum frequency is set to the maximum supported
by the peripheral, not the controller, see shields for example.

On the occasion of bringing it back, the original function was
refactored to be easier to read and understand.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>
Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>