[LPC1549] Improve AnalogIn Performance

Modified the LPC1549 AnalogIn implementation to use the ADCs in
synchronous mode, which is consistent with the LPC11U68 implementation.
This improves performance, and allows the IRC oscillator to be powered
down if necessary.

Author: neilt6

libraries/mbed/targets/hal/TARGET_NXP/TARGET_LPC15XX/analogin_api.c

@@ -77,10 +77,6 @@ void analogin_init(analogin_t *obj, PinName pin) {
         LPC_SYSCON->SYSAHBCLKCTRL0 |= (1 << 28);
     }
 
-    // select IRC as asynchronous clock, divided by 1
-    LPC_SYSCON->ADCASYNCCLKSEL  = 0;
-    LPC_SYSCON->ADCASYNCCLKDIV  = 1;
-
     __IO LPC_ADC0_Type *adc_reg = (obj->adc < ADC1_0) ? (__IO LPC_ADC0_Type*)(LPC_ADC0) : (__IO LPC_ADC0_Type*)(LPC_ADC1);
 
     // determine the system clock divider for a 500kHz ADC clock during calibration
@@ -90,8 +86,8 @@ void analogin_init(analogin_t *obj, PinName pin) {
     adc_reg->CTRL = (1UL << 30) | (clkdiv & 0xFF);
     while ((adc_reg->CTRL & (1UL << 30)) != 0);
 
-    // switch to asynchronous mode
-    adc_reg->CTRL = (1UL << 8);
+    // Sampling clock: SystemClock divided by 1
+    adc_reg->CTRL = 0;
 }
 
 static inline uint32_t adc_read(analogin_t *obj) {