boards: disco_l475_iot1: fix arduino_i2c config

disco l475 board exposes I2C3 on standard Arduino Uno pins A4/A5,
not I2C1.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>
(cherry picked from commit 0f05f58)

Author: kartben

boards/st/disco_l475_iot1/arduino_r3_connector.dtsi

@@ -35,6 +35,6 @@
 	};
 };
 
-arduino_i2c: &i2c1 {};
+arduino_i2c: &i2c3 {};
 arduino_spi: &spi1 {};
 arduino_serial: &uart4 {};