kinetis: Filter out PWM code if no FTM exists

cpu/kinetis/periph/pwm.c

@@ -29,6 +29,12 @@
 #include "assert.h"
 #include "periph/pwm.h"
 
+/* This driver uses the FlexTimer module (FTM) for generating PWM signals, but
+ * not all Kinetis CPUs have such a module. This preprocessor condition prevents
+ * compilation errors when the CPU header lacks the register definitions for the
+ * FTM */
+#ifdef FTM0
+
 #define PRESCALER_MAX       (7U)
 
 static inline FTM_Type *ftm(pwm_t pwm)
@@ -150,3 +156,5 @@ void pwm_poweroff(pwm_t pwm)
     }
 #endif
 }
+
+#endif /* defined(FTM0) */