Added IIC pins (#1436)

Added IIC pins. I can confirm that this is working!

variants/esp32-gateway/pins_arduino.h

@@ -16,6 +16,10 @@ static const uint8_t LED_BUILTIN = 33;
 
 static const uint8_t KEY_BUILTIN = 34;
 
+// See https://github.com/OLIMEX/ESP32-GATEWAY/blob/master/SOFTWARE/MOD-RTC/main/main.c for more info about IIC on the GATEWAY
+static const uint8_t SCL = 16; // This is pin 11
+static const uint8_t SDA = 17; // This is pin 12
+
 static const uint8_t TX = 1;
 static const uint8_t RX = 3;