i2c: implement target mode for rp2040 and nrf

src/machine/machine_nrf5x.go

@@ -6,7 +6,8 @@ import "device/nrf"
 
 // I2C on the NRF51 and NRF52.
 type I2C struct {
-	Bus *nrf.TWI_Type
+	Bus  *nrf.TWI_Type
+	mode I2CMode
 }
 
 // There are 2 I2C interfaces on the NRF.
@@ -19,6 +20,10 @@ func (i2c *I2C) enableAsController() {
 	i2c.Bus.ENABLE.Set(nrf.TWI_ENABLE_ENABLE_Enabled)
 }
 
+func (i2c *I2C) enableAsTarget() {
+	// Not supported on this hardware
+}
+
 func (i2c *I2C) disable() {
 	i2c.Bus.ENABLE.Set(0)
 }