mfd: syscon: Add missing struct device_node declaration

syscon.h header uses struct device_node in functions' declarations
without declaring it. This causes compilation warnings like:

include/linux/mfd/syscon.h:20: warning: ‘struct device_node’
declared inside parameter list
include/linux/mfd/syscon.h:20: warning: its scope is only this
definition or declaration, which is probably not what you want

Fix it by adding a forward declaration of struct device_node.
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

include/linux/mfd/syscon.h

@@ -15,6 +15,8 @@
 #ifndef __LINUX_MFD_SYSCON_H__
 #define __LINUX_MFD_SYSCON_H__
 
+struct device_node;
+
 extern struct regmap *syscon_node_to_regmap(struct device_node *np);
 extern struct regmap *syscon_regmap_lookup_by_compatible(const char *s);
 extern struct regmap *syscon_regmap_lookup_by_pdevname(const char *s);