spi: Refactor mcux dspi shim driver to use clock control interface

Refactors the mcux dspi shim driver to use the clock control interface
instead of calling CLOCK_GetFreq() directly. With this change, we are
now getting all soc-specific information from device tree and can remove
the direct dependency on soc.h.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>