targets: add implementation for Tillitis TKey device (#4631)

* initial implementation for Tillitis TKey device
* add UART implementation for TKey
* add Pin interface implementation for TKey touch sensor
* add RNG interface implementation for TKey
* add helpful machine package functions to return identifiers such as name and version for TKey
* use built-in timer for sleep timing on TKey
* modify UART implementation for TKey to implement Serialer interface
* implement BLAKE2s ROM function call for TKey device
* handle abort by triggering TKey device fault using illegal instruction to halt CPU
* simplify TKey implementation by inheriting from existing riscv32 target
* return error for trying to configure invalid baudrates on UART
* add tkey to builder test
* be very specific for features passed to LLVM for specific config in use for TKey
* handle feedback items from TKey device code review

Signed-off-by: deadprogram <ron@hybridgroup.com>

src/runtime/runtime_tkey.go

@@ -0,0 +1,64 @@
+//go:build tkey
+
+// This file implements target-specific things for the TKey.
+
+package runtime
+
+import (
+	"device/tkey"
+	"machine"
+	"runtime/volatile"
+)
+
+type timeUnit int64
+
+//export main
+func main() {
+	preinit()
+	initPeripherals()
+	run()
+	exit(0)
+}
+
+// initPeripherals configures peripherals the way the runtime expects them.
+func initPeripherals() {
+	// prescaler value that results in 0.00001-second timer-ticks.
+	// given an 18 MHz processor, a millisecond is about 18,000 cycles.
+	tkey.TIMER.PRESCALER.Set(18 * machine.MHz / 100000)
+
+	machine.InitSerial()
+}
+
+func putchar(c byte) {
+	machine.Serial.WriteByte(c)
+}
+
+func getchar() byte {
+	for machine.Serial.Buffered() == 0 {
+		Gosched()
+	}
+	v, _ := machine.Serial.ReadByte()
+	return v
+}
+
+func buffered() int {
+	return machine.Serial.Buffered()
+}
+
+var timestamp volatile.Register32
+
+// ticks returns the current value of the timer in ticks.
+func ticks() timeUnit {
+	return timeUnit(timestamp.Get())
+}
+
+// sleepTicks sleeps for at least the duration d.
+func sleepTicks(d timeUnit) {
+	target := uint32(ticks() + d)
+
+	tkey.TIMER.TIMER.Set(uint32(d))
+	tkey.TIMER.CTRL.SetBits(tkey.TK1_MMIO_TIMER_CTRL_START)
+	for tkey.TIMER.STATUS.Get() != 0 {
+	}
+	timestamp.Set(target)
+}