machine: add simulated PWM/timer peripherals

I will soon use these as part of the TinyGo tour, to explain the PWM
peripherals on common chips.

src/machine/machine_nrf52840_simulator.go

@@ -0,0 +1,60 @@
+//go:build !baremetal && (bluemicro840 || circuitplay_bluefruit || clue_alpha || feather_nrf52840_sense || feather_nrf52840 || itsybitsy_nrf52840 || mdbt50qrx || nano_33_ble || nicenano || nrf52840_mdk || particle_3rd_gen || pca10056 || pca10059 || rak4631 || reelboard || xiao_ble)
+
+// Simulator support for nrf52840 based boards.
+
+package machine
+
+// Channel values below are nil, so that they get filled in on the first use.
+// This is the same as what happens on baremetal.
+
+var PWM0 = timerType{
+	instance:   0,
+	frequency:  16e6,
+	bits:       15,
+	prescalers: []int{1, 2, 4, 8, 16, 32, 64, 128},
+	channelPins: [][]Pin{
+		nil, // channel 0
+		nil, // channel 1
+		nil, // channel 2
+		nil, // channel 3
+	},
+}
+
+var PWM1 = timerType{
+	instance:   1,
+	frequency:  16e6,
+	bits:       15,
+	prescalers: []int{1, 2, 4, 8, 16, 32, 64, 128},
+	channelPins: [][]Pin{
+		nil, // channel 0
+		nil, // channel 1
+		nil, // channel 2
+		nil, // channel 3
+	},
+}
+
+var PWM2 = timerType{
+	instance:   2,
+	frequency:  16e6,
+	bits:       15,
+	prescalers: []int{1, 2, 4, 8, 16, 32, 64, 128},
+	channelPins: [][]Pin{
+		nil, // channel 0
+		nil, // channel 1
+		nil, // channel 2
+		nil, // channel 3
+	},
+}
+
+var PWM3 = timerType{
+	instance:   3,
+	frequency:  16e6,
+	bits:       15,
+	prescalers: []int{1, 2, 4, 8, 16, 32, 64, 128},
+	channelPins: [][]Pin{
+		nil, // channel 0
+		nil, // channel 1
+		nil, // channel 2
+		nil, // channel 3
+	},
+}