nrf: fix memory issue in ADC read

There was a very subtle bug in the ADC read code: it stores a pointer to
a variable in a register, waits for the hardware to complete the read,
and then reads the value again from the local variable. Unfortunately,
the compiler doesn't know there is some form of synchronization
happening in between.

This can be fixed in roughly two ways:
  * Introduce some sort of synchronization.
  * Do a volatile read from the variable.

I chose the second one as it is probably the least intrusive. We
certainly don't need atomic instructions (the chip is single threaded),
we just need to tell the compiler the value could have changed by making
the read volatile.

src/machine/machine_nrf528xx.go

@@ -25,7 +25,7 @@ func (a ADC) Configure(ADCConfig) {
 // Get returns the current value of a ADC pin in the range 0..0xffff.
 func (a ADC) Get() uint16 {
 	var pwmPin uint32
-	var value int16
+	var rawValue volatile.Register16
 
 	switch a.Pin {
 	case 2:
@@ -78,7 +78,7 @@ func (a ADC) Get() uint16 {
 	nrf.SAADC.CH[0].PSELP.Set(pwmPin)
 
 	// Destination for sample result.
-	nrf.SAADC.RESULT.PTR.Set(uint32(uintptr(unsafe.Pointer(&value))))
+	nrf.SAADC.RESULT.PTR.Set(uint32(uintptr(unsafe.Pointer(&rawValue))))
 	nrf.SAADC.RESULT.MAXCNT.Set(1) // One sample
 
 	// Start tasks.
@@ -104,6 +104,7 @@ func (a ADC) Get() uint16 {
 	// Disable the ADC.
 	nrf.SAADC.ENABLE.Set(nrf.SAADC_ENABLE_ENABLE_Disabled << nrf.SAADC_ENABLE_ENABLE_Pos)
 
+	value := int16(rawValue.Get())
 	if value < 0 {
 		value = 0
 	}