Support other CPU speeds in ADC & DAC

gui/list.html

 		C0G/NPO ceramic capacitor should be used for filtering.  Low
 		quality ceramic (X7R, Y5V, Z5U, etc) can cause signal distortion.</p>
 	<h3>Notes</h3>
+	<p>This object only works properly when Tools > CPU_Speed is set to
+		48 or 96 MHz.  Other speeds aren't supported and will likely fail
+		in strange ways.</p>
 	<p>The PWM carrier frequency is 88.2 kHz.  The suggested circuit
 		will only slightly filter the carrier.  Extra filtering will be
 		required for a clean signal without the ultrasonic PWM carrier.

output_pwm.cpp

 DMAMEM uint32_t pwm_dma_buffer[AUDIO_BLOCK_SAMPLES*2];
 DMAChannel AudioOutputPWM::dma;
 
+// TODO: this code assumes F_BUS is 48 MHz.
+// supporting other speeds is not easy, but should be done someday
+
 void AudioOutputPWM::begin(void)
 {
 	dma.begin(true); // Allocate the DMA channel first

utility/pdb.h

 #ifndef pdb_h_
 #define pdb_h_
 
+#include "kinetis.h"
+
 // Multiple input & output objects use the Programmable Delay Block
 // to set their sample rate.  They must all configure the same
 // period to avoid chaos.
 
 #define PDB_CONFIG (PDB_SC_TRGSEL(15) | PDB_SC_PDBEN | PDB_SC_CONT)
-#define PDB_PERIOD 1087 // 48e6 / 44100
+
+#if F_BUS == 60000000
+  #define PDB_PERIOD (1360-1)
+#elif F_BUS == 56000000
+  #define PDB_PERIOD (1269-1)  // 0.026% error
+#elif F_BUS == 48000000
+  #define PDB_PERIOD (1088-1)
+#elif F_BUS == 36000000
+  #define PDB_PERIOD (816-1)
+#elif F_BUS == 24000000
+  #define PDB_PERIOD (544-1)
+#elif F_BUS == 16000000
+  #define PDB_PERIOD (363-1)  // 0.092% error
+#else
+  #error "Unsupported F_BUS speed"
+#endif
 
 #endif