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/cm_fx6.h b/include/configs/cm_fx6.h
index 4207504..d6e5a2b 100644
--- a/include/configs/cm_fx6.h
+++ b/include/configs/cm_fx6.h
@@ -245,6 +245,7 @@
 #define CONFIG_CMD_I2C
 #define CONFIG_SYS_I2C
 #define CONFIG_SYS_I2C_MXC
+#define CONFIG_SYS_I2C_MXC_I2C3		/* enable I2C bus 3 */
 #define CONFIG_SYS_I2C_SPEED		100000
 #define CONFIG_SYS_MXC_I2C3_SPEED	400000