ws2812: add nrf52 support, update timings for m0 and avr

This commit adds support for the nrf52, which is a Cortex-M4 running at
64MHz.
While implementing this, I discovered that the timings for AVR/16MHz and
Cortex-M0 at 16MHz weren't quite right. They do work, but were slightly
out of spec. So I fixed this as well.

ws2812/ws2812_m0_16m.go

@@ -15,23 +15,29 @@ func (d Device) WriteByte(c byte) error {
 	portSet, maskSet := d.Pin.PortMaskSet()
 	portClear, maskClear := d.Pin.PortMaskClear()
 
+	// See:
+	// https://wp.josh.com/2014/05/13/ws2812-neopixels-are-not-so-finicky-once-you-get-to-know-them/
+	// T0H: 4  cycles or 250ns
+	// T0L: 14 cycles or 875ns -> together 18 cycles or 1125ns
+	// T1H: 10 cycles or 625ns
+	// T1H: 8  cycles or 500ns -> together 18 cycles or 1125ns
 	value := uint32(c) << 24
 	arm.AsmFull(`
 	send_bit:
-		str   {maskSet}, {portSet}
-		lsls  {value}, #1
-		bcs.n skip_store
-		str   {maskClear}, {portClear}
+		str   {maskSet}, {portSet}     @ [2]   T0H and T0L start here
+		lsls  {value}, #1              @ [1]
+		bcs.n skip_store               @ [1/3]
+		str   {maskClear}, {portClear} @ [2]   T0H -> T0L transition
 	skip_store:
+		nop                            @ [4]
 		nop
 		nop
 		nop
+		str   {maskClear}, {portClear} @ [2]   T1H -> T1L transition
+		nop                            @ [2]
 		nop
-		nop
-		nop
-		str   {maskClear}, {portClear}
-		subs  {i}, #1
-		bne.n send_bit
+		subs  {i}, #1                  @ [1]
+		bne.n send_bit                 @ [1/3]
 	`, map[string]interface{}{
 		"value":     value,
 		"i":         8,