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/README b/README
index fc1fd52..d33f3b5 100644
--- a/README
+++ b/README
@@ -2395,6 +2395,8 @@
 		  - define slave for bus 3 with CONFIG_SYS_MXC_I2C3_SLAVE
 		If those defines are not set, default value is 100000
 		for speed, and 0 for slave.
+		  - enable bus 3 with CONFIG_SYS_I2C_MXC_I2C3
+		  - enable bus 4 with CONFIG_SYS_I2C_MXC_I2C4
 
 		- drivers/i2c/rcar_i2c.c:
 		  - activate this driver with CONFIG_SYS_I2C_RCAR