drivers: spi_nrfx_spim: bring back get_nrf_spim_frequency

Commit 246393e
("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>

Author: jfischer-no

drivers/spi/spi_nrfx_spim.c

@@ -57,6 +57,31 @@ struct spi_nrfx_config {
 
 static void event_handler(const nrfx_spim_evt_t *p_event, void *p_context);
 
+static inline uint32_t get_nrf_spim_frequency(uint32_t frequency)
+{
+	/* Get the highest supported frequency not exceeding the requested one.
+	 */
+	if (frequency >= MHZ(32) && NRF_SPIM_HAS_32_MHZ_FREQ) {
+		return MHZ(32);
+	} else if (frequency >= MHZ(16) && NRF_SPIM_HAS_16_MHZ_FREQ) {
+		return MHZ(16);
+	} else if (frequency >= MHZ(8)) {
+		return MHZ(8);
+	} else if (frequency >= MHZ(4)) {
+		return MHZ(4);
+	} else if (frequency >= MHZ(2)) {
+		return MHZ(2);
+	} else if (frequency >= MHZ(1)) {
+		return MHZ(1);
+	} else if (frequency >= KHZ(500)) {
+		return KHZ(500);
+	} else if (frequency >= KHZ(250)) {
+		return KHZ(250);
+	} else {
+		return KHZ(125);
+	}
+}
+
 static inline nrf_spim_mode_t get_nrf_spim_mode(uint16_t operation)
 {
 	if (SPI_MODE_GET(operation) & SPI_MODE_CPOL) {
@@ -143,7 +168,8 @@ static int configure(const struct device *dev,
 	config = dev_config->def_config;
 
 	/* Limit the frequency to that supported by the SPIM instance. */
-	config.frequency = MIN(spi_cfg->frequency, max_freq);
+	config.frequency = get_nrf_spim_frequency(MIN(spi_cfg->frequency,
+						      max_freq));
 	config.mode      = get_nrf_spim_mode(spi_cfg->operation);
 	config.bit_order = get_nrf_spim_bit_order(spi_cfg->operation);