nrf: don't block SPI transfer

Ideally we'd use something like
#5016 but that's a bit more
involved. As a quick improvement, call gosched() instead.

Example where I use this: a custom WS2812 driver that uses SPI to
transfer the data. It's useful to be able to switch back to the main
goroutine during the transfer to render the next LED update.

Author: aykevl

src/machine/machine_nrf52xxx.go

@@ -335,6 +335,7 @@ func (spi *SPI) Tx(w, r []byte) error {
 		// finished if the transfer is send-only (a common case).
 		spi.Bus.TASKS_START.Set(1)
 		for spi.Bus.EVENTS_END.Get() == 0 {
+			gosched()
 		}
 		spi.Bus.EVENTS_END.Set(0)
 	}