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/aristainetos.h b/include/configs/aristainetos.h
index 3066fd0..cc26790 100644
--- a/include/configs/aristainetos.h
+++ b/include/configs/aristainetos.h
@@ -262,6 +262,7 @@
 #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_I2C_SLAVE		0x7f
 #define CONFIG_SYS_I2C_NOPROBES		{ {0, 0x00} }