of: Add stub of_get_parent for non-OF builds

Fixes build error on x86_64 allmodconfig, introduced by commit
5ab3a89 ("mfd: syscon: Add non-DT support").

drivers/regulator/anatop-regulator.c: In function 'anatop_regulator_probe':
drivers/regulator/anatop-regulator.c:134:2: error: implicit declaration of function 'of_get_parent' [-Werror=implicit-function-declaration]

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

include/linux/of.h

@@ -353,6 +353,11 @@ static inline struct device_node *of_find_node_by_name(struct device_node *from,
 	return NULL;
 }
 
+static inline struct device_node *of_get_parent(const struct device_node *node)
+{
+	return NULL;
+}
+
 static inline bool of_have_populated_dt(void)
 {
 	return false;