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: Nkbuild test robot <fengguang.wu@intel.com>
Cc: Dave Gerlach <d-gerlach@ti.com>
Cc: Suman Anna <s-anna@ti.com>
Signed-off-by: NOhad Ben-Cohen <ohad@wizery.com>

drivers/remoteproc/remoteproc_core.c

@@ -1178,6 +1178,7 @@ EXPORT_SYMBOL(rproc_shutdown);
  *
  * 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;
@@ -1201,6 +1202,12 @@ struct rproc *rproc_get_by_phandle(phandle phandle)
 
 	return rproc;
 }
+#else
+struct rproc *rproc_get_by_phandle(phandle phandle)
+{
+	return NULL;
+}
+#endif
 EXPORT_SYMBOL(rproc_get_by_phandle);
 
 /**