drivers: spi: gecko: Respect frequency configuration

This commit adds support for configuring SPI frequency per transaction.
The "clock-frequency" devicetree property is used as the default
frequency unless the peripheral being communicated with has a lower
"max-spi-frequency" (passed to the driver as spi_config::frequency).

An error is returned if the requested frequency is higher than the
hardware can support. This limit is half the peripheral clock (PCLK on
Series 2, HFPERCLK on Series 0/1) for SPI in controller mode.

Previously, the driver hard-coded 1 MHz operation. For backwards
compatibility, default to 1 MHz if the "clock-frequency" property is not
set in devicetree.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>