runtime/nrf: Add Sleep()

src/runtime/runtime_nrf.go

@@ -8,6 +8,7 @@ import "C"
 
 func init() {
 	C.uart_init(6) // pin_tx = 6, for NRF52840-DK
+	C.rtc_init()
 }
 
 const Microsecond = 1
@@ -17,7 +18,7 @@ func putchar(c byte) {
 }
 
 func Sleep(d Duration) {
-	// TODO
+	C.rtc_sleep(C.uint32_t(d / 32)) // TODO: not accurate (must be d / 30.5175...)
 }
 
 func abort() {