Update HardwareSerial.h Adding 9N1 support

I added two define statements to allow boards like the UNO R4 WIFI to use their 9-bit UART via the 16-bit register. This is to support a pull request coming to the renesas_uno core to activate and facilitate the processor's 9-bit functionality.

api/HardwareSerial.h

@@ -42,12 +42,14 @@ namespace arduino {
 #define SERIAL_DATA_6        (0x200ul)
 #define SERIAL_DATA_7        (0x300ul)
 #define SERIAL_DATA_8        (0x400ul)
+#define SERIAL_DATA_9        (0x500ul)
 #define SERIAL_DATA_MASK     (0xF00ul)
 
 #define SERIAL_5N1           (SERIAL_STOP_BIT_1 | SERIAL_PARITY_NONE  | SERIAL_DATA_5)
 #define SERIAL_6N1           (SERIAL_STOP_BIT_1 | SERIAL_PARITY_NONE  | SERIAL_DATA_6)
 #define SERIAL_7N1           (SERIAL_STOP_BIT_1 | SERIAL_PARITY_NONE  | SERIAL_DATA_7)
 #define SERIAL_8N1           (SERIAL_STOP_BIT_1 | SERIAL_PARITY_NONE  | SERIAL_DATA_8)
+#define SERIAL_9N1           (SERIAL_STOP_BIT_1 | SERIAL_PARITY_NONE  | SERIAL_DATA_9)
 #define SERIAL_5N2           (SERIAL_STOP_BIT_2 | SERIAL_PARITY_NONE  | SERIAL_DATA_5)
 #define SERIAL_6N2           (SERIAL_STOP_BIT_2 | SERIAL_PARITY_NONE  | SERIAL_DATA_6)
 #define SERIAL_7N2           (SERIAL_STOP_BIT_2 | SERIAL_PARITY_NONE  | SERIAL_DATA_7)
@@ -103,4 +105,4 @@ class HardwareSerial : public Stream
 // XXX: Are we keeping the serialEvent API?
 extern void serialEventRun(void) __attribute__((weak));
 
-}
+}