driver/i2c/mxc: Enable I2C bus 3 and 4

Some SoCs have more than two I2C busses. Instead of adding ifdef
to the driver, macros are put into board header file where
CONFIG_SYS_I2C_MXC is defined.

Signed-off-by: York Sun <yorksun@freescale.com>
CC: Heiko Schocher <hs@denx.de>
diff --git a/include/configs/flea3.h b/include/configs/flea3.h
index 854ae90..5f7cad8 100644
--- a/include/configs/flea3.h
+++ b/include/configs/flea3.h
@@ -52,6 +52,7 @@
  */
 #define CONFIG_SYS_I2C
 #define CONFIG_SYS_I2C_MXC
+#define CONFIG_SYS_I2C_MXC_I2C3		/* enable I2C bus 3 */
 #define CONFIG_SYS_SPD_BUS_NUM		2 /* I2C3 */
 #define CONFIG_SYS_MXC_I2C3_SLAVE	0xfe
 #define CONFIG_MXC_SPI