machine: add dummy async SPI methods

This prepares all devices for actual async SPI support.

src/machine/spi-dummy-async.go

@@ -0,0 +1,29 @@
+//go:build !baremetal || atmega || esp32 || fe310 || k210 || nrf || (nxp && !mk66f18) || rp2040 || sam || (stm32 && !stm32f7x2 && !stm32l5x2)
+
+package machine
+
+// This is a non-async implementation of the async SPI calls.
+// It is useful for devices that don't support DMA on SPI, or for which it
+// hasn't been implemented yet.
+
+// IsAsync returns whether the SPI supports async operation (usually DMA).
+//
+// This SPI does not support async operations.
+func (s SPI) IsAsync() bool {
+	return false
+}
+
+// Start a transfer in the background.
+//
+// Because this SPI implementation doesn't support async operation, it is an
+// alias for Tx.
+func (s SPI) StartTx(tx, rx []byte) error {
+	return s.Tx(tx, rx)
+}
+
+// Wait until all active transactions (started by StartTx) have finished.
+//
+// This is a no-op on this SPI implementation.
+func (s SPI) Wait() error {
+	return nil
+}

src/machine/spi.go

@@ -26,4 +26,7 @@ var _ interface { // 2
 	Configure(config SPIConfig) error
 	Tx(w, r []byte) error
 	Transfer(w byte) (byte, error)
+	IsAsync() bool
+	StartTx(tx, rx []byte) error
+	Wait() error
 } = (*SPI)(nil)