remoteproc: fix !CONFIG_OF build breakage

Fix this:

drivers/remoteproc/remoteproc_core.c: In function 'rproc_get_by_phandle':
>> drivers/remoteproc/remoteproc_core.c:1167:2: error: implicit declaration of function 'of_find_node_by_phandle' [-Werror=implicit-function-declaration]
np = of_find_node_by_phandle(phandle);

Reported-by: kbuild test robot <[e-mail]>
Cc: Dave Gerlach <[e-mail]>
Cc: Suman Anna <[e-mail]>
Signed-off-by: Ohad Ben-Cohen <[e-mail]>

drivers/remoteproc/remoteproc_core.c

···
  *
  * Returns the rproc handle on success, and NULL on failure.
  */
+#ifdef CONFIG_OF
 struct rproc *rproc_get_by_phandle(phandle phandle)
 {
 	struct rproc *rproc = NULL, *r;
···
 
 	return rproc;
 }
+#else
+struct rproc *rproc_get_by_phandle(phandle phandle)
+{
+	return NULL;
+}
+#endif
 EXPORT_SYMBOL(rproc_get_by_phandle);
 
 /**