spi/fsl-espi: Make sure pm is within 2..32

The reference manual says that pm has to stay within 2 and 32. So the
lowest frequency is 32 and DIV16 set, the highest is 2 and DIV16 unset.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>