ws2812: add support for ESP8266

This patch adds support for the ESP8266 chip by adding support for 80MHz
operation. This should also work when the ESP32 is changed to 80MHz, but
I didn't test it (as there is not currently a way to do that).

Verified on a NodeMCU dev board with an ESP8266.

ws2812/ws2812_xtensa.go

@@ -16,7 +16,7 @@ func (d Device) WriteByte(c byte) error {
 	portClear, maskClear := d.Pin.PortMaskClear()
 
 	switch machine.CPUFrequency() {
-	case 160e6:
+	case 160e6: // 160MHz
 		// See:
 		// https://wp.josh.com/2014/05/13/ws2812-neopixels-are-not-so-finicky-once-you-get-to-know-them/
 		// Because I do not know the exact instruction timings, I'm going to
@@ -224,6 +224,122 @@ func (d Device) WriteByte(c byte) error {
 			"portClear": uintptr(unsafe.Pointer(portClear)),
 		})
 		return nil
+	case 80e6: // 80MHz
+		// See docs for 160MHz.
+		// T0H: 21 cycles or  262.5ns
+		// T0L: 67 cycles or  837.5ns
+		//   +: 88 cycles or 1100.0ns
+		// T1H: 47 cycles or  587.5ns
+		// T1L: 39 cycles or  487.5ns
+		//   +: 86 cycles or 1075.0ns
+		device.AsmFull(`
+		1: // send_bit
+			s32i  {maskSet}, {portSet}, 0     // [1]  T0H and T1H start here
+			nop                               // [18]
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			slli  {value}, {value}, 1         // [1]  shift {value} to the left by 1
+			bbsi  {value}, 8, 2f              // [1]  branch to skip_store if bit 8 is set
+			s32i  {maskClear}, {portClear}, 0 // [1]  T0H -> T0L transition
+		2: // skip_store
+			nop                               // [27]
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			s32i  {maskClear}, {portClear}, 0 // [1]  T1H -> T1L transition
+			nop                               // [36]
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			nop
+			addi  {i}, {i}, -1                // [1]
+			bnez {i}, 1b                      // [1]  send_bit, T1H and T1L end here
+
+			// Restore original values after modifying them in the inline
+			// assembly. Not doing that would result in undefined behavior as
+			// the compiler doesn't know we're modifying these values.
+			movi.n {i}, 8
+			slli  {value}, {value}, 8
+		`, map[string]interface{}{
+			// Note: casting pointers to uintptr here because of what might be
+			// an Xtensa backend bug with inline assembly.
+			"value":     uint32(c),
+			"i":         8,
+			"maskSet":   maskSet,
+			"portSet":   uintptr(unsafe.Pointer(portSet)),
+			"maskClear": maskClear,
+			"portClear": uintptr(unsafe.Pointer(portClear)),
+		})
+		return nil
 	default:
 		return errUnknownClockSpeed
 	}