designware_i2c.h: Define IC_CLK only if not already defined in config file

Signed-off-by: Armando Visconti <armando.visconti@st.com>

drivers/i2c/designware_i2c.h

@@ -60,7 +60,9 @@ struct i2c_regs {
 	u32 ic_tx_abrt_source;
 };
 
+#if !defined(IC_CLK)
 #define IC_CLK			166
+#endif
 #define NANO_TO_MICRO		1000
 
 /* High and low times in different speed modes (in ns) */