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>