drivers: i2c: mxc: Fix compiler error when using i2c dm mode

I2C dm mode enablemenet causes below compilation errors:

In file included from include/config.h:8:0,
                 from include/common.h:20:
include/config_fallbacks.h:51:4: error: #error "Cannot define
CONFIG_SYS_I2C when CONFIG_DM_I2C is used"
 #  error "Cannot define CONFIG_SYS_I2C when CONFIG_DM_I2C is used"
    ^~~~~
In file included from include/config.h:8:0,
                 from include/common.h:20:
include/config_fallbacks.h:51:4: error: #error "Cannot define
CONFIG_SYS_I2C when CONFIG_DM_I2C is used"
 #  error "Cannot define CONFIG_SYS_I2C when CONFIG_DM_I2C is used"
    ^~~~~

board/freescale/lx2160a/lx2160a.c: In function 'board_early_init_f':
board/freescale/lx2160a/lx2160a.c:108:2: warning: implicit declaration
of function 'i2c_early_init_f'; did you mean 'arch_early_init_r'?
[-Wimplicit-function-declaration]
  i2c_early_init_f();
  ^~~~~~~~~~~~~~~~
  arch_early_init_r

 drivers/i2c/mxc_i2c.c: In function 'mxc_i2c_probe':
  drivers/i2c/mxc_i2c.c:824:8: warning: implicit declaration of function
'enable_i2c_clk';
  did you mean 'enable_irq_wake'? [-Wimplicit-function-declaration]
  ret = enable_i2c_clk(1, bus->seq);
        ^~~~~~~~~~~~~~
        enable_irq_wake

So fix these compilation errors.

Signed-off-by: Chuanhua Han <chuanhua.han@nxp.com>
Reviewed-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
diff --git a/drivers/i2c/mxc_i2c.c b/drivers/i2c/mxc_i2c.c
index 23119cc..73b9807 100644
--- a/drivers/i2c/mxc_i2c.c
+++ b/drivers/i2c/mxc_i2c.c
@@ -558,6 +558,14 @@
 	return 0;
 }
 
+int __enable_i2c_clk(unsigned char enable, unsigned int i2c_num)
+{
+	return 1;
+}
+
+int enable_i2c_clk(unsigned char enable, unsigned int i2c_num)
+	__attribute__((weak, alias("__enable_i2c_clk")));
+
 #ifndef CONFIG_DM_I2C
 /*
  * Read data from I2C device
@@ -723,13 +731,6 @@
 	return bus_i2c_write(i2c_get_base(adap), chip, 0, 0, NULL, 0);
 }
 
-int __enable_i2c_clk(unsigned char enable, unsigned i2c_num)
-{
-	return 1;
-}
-int enable_i2c_clk(unsigned char enable, unsigned i2c_num)
-	__attribute__((weak, alias("__enable_i2c_clk")));
-
 void bus_i2c_init(int index, int speed, int unused,
 		  int (*idle_bus_fn)(void *p), void *idle_bus_data)
 {