T4 - setTX with open drain mode.

Allow the user to call setTX with the one hardware pin, and hopefully allow them to turn on Open Drain mode or turn it back off.  I don't currently have any test cases that use this, so not sure how well it works.

teensy4/HardwareSerial.cpp

@@ -236,8 +236,14 @@ void HardwareSerial::setRX(uint8_t pin)
 
 void HardwareSerial::setTX(uint8_t pin, bool opendrain)
 {
-	// Currently none of these have multiple 
-	// possible TX pins
+	// While all of our TX pins only have one defined pin, we can choose to 
+	// turn on or off opendrain mode.
+	if (pin == hardware->tx_pin) {
+		if (opendrain) 
+			*(portControlRegister(hardware->tx_pin)) = IOMUXC_PAD_ODE | IOMUXC_PAD_DSE(3) | IOMUXC_PAD_SPEED(3);
+		else 	
+			*(portControlRegister(hardware->tx_pin)) = IOMUXC_PAD_SRE | IOMUXC_PAD_DSE(3) | IOMUXC_PAD_SPEED(3);
+	}
 }