drivers: clock_control: nxp: Fix building MCG on KW40Z/KW41Z

The KW40Z/KW41Z don't have a Fixed Freq MCG clock so the code
associated with that in get_rate fails to build.  ifdef around
the code with enum kCLOCK_McgFixedFreqClk so things work
correctly.

Signed-off-by: Kumar Gala <galak@kernel.org>

drivers/clock_control/clock_control_mcux_mcg.c

@@ -37,9 +37,11 @@ static int mcux_mcg_get_rate(const struct device *dev,
 	clock_name_t clock_name;
 
 	switch ((uint32_t) sub_system) {
+#ifdef kCLOCK_McgFixedFreqClk
 	case KINETIS_MCG_FIXED_FREQ_CLK:
 		clock_name = kCLOCK_McgFixedFreqClk;
 		break;
+#endif
 	case KINETIS_MCG_OUT_CLK:
 		*rate = CLOCK_GetOutClkFreq();
 		return 0;