clk: mediatek: Fix all warnings for missing struct clk_onecell_data

Even though the header file linux/clk-provider.h is already being properly
included in clk-mtk.h, the definition of struct clk_onecell_data still
must depend on CONFIG_COMMON_CLK defined and thus it's possible that
below build warnings occur when CONFIG_COMMON_CLK is not being selected.

Therefore, these functions which need struct clk_onecell_data without
declaring that structure first requires simply declaring that this struct
exists prior to referencing it in clk-mtk.h

Changes from v1->v2:
enhance v1 based on two useful solutions Jean Delvare kindly suggested.

All warnings (new ones prefixed by >>):

In file included from drivers/clk/mediatek/reset.c:22:0:
>>drivers/clk/mediatek/clk-mtk.h:44:19: warning: 'struct clk_onecell_data'
declared inside parameter list will not be visible outside of
this definition or declaration
  int num, struct clk_onecell_data *clk_data);
		  ^~~~~~~~~~~~~~~~
drivers/clk/mediatek/clk-mtk.h:63:19: warning: 'struct clk_onecell_data'
declared inside parameter list will not be visible outside of
this definition or declaration
  int num, struct clk_onecell_data *clk_data);
		  ^~~~~~~~~~~~~~~~
drivers/clk/mediatek/clk-mtk.h:145:10: warning: 'struct clk_onecell_data'
declared inside parameter list will not be visible outside of
this definition or declaration
  struct clk_onecell_data *clk_data);
	 ^~~~~~~~~~~~~~~~
drivers/clk/mediatek/clk-mtk.h:164:11: warning: 'struct clk_onecell_data'
declared inside parameter list will not be visible outside of
this definition or declaration
   struct clk_onecell_data *clk_data);
	  ^~~~~~~~~~~~~~~~
drivers/clk/mediatek/clk-mtk.h:190:12: warning: 'struct clk_onecell_data'
declared inside parameter list will not be visible outside of this
definition or declaration
    struct clk_onecell_data *clk_data);
	   ^~~~~~~~~~~~~~~~

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Cc: kbuild-all@01.org
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Jean Delvare <jdelvare@suse.de>
Cc: linux-clk@vger.kernel.org
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>

drivers/clk/mediatek/clk-mtk.h

@@ -20,6 +20,7 @@
 #include <linux/clk-provider.h>
 
 struct clk;
+struct clk_onecell_data;
 
 #define MAX_MUX_GATE_BIT	31
 #define INVALID_MUX_GATE_BIT	(MAX_MUX_GATE_BIT + 1)