dm: core: Use dev_has_ofnode() instead of dev_of_valid()

We have two functions which do the same thing. Standardise on
dev_has_ofnode() since there is no such thing as an 'invalid' ofnode in
normal operation: it is either null or missing.

Also move the functions into one place.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
diff --git a/drivers/sysreset/sysreset_sandbox.c b/drivers/sysreset/sysreset_sandbox.c
index 7026a48..8eca7d8 100644
--- a/drivers/sysreset/sysreset_sandbox.c
+++ b/drivers/sysreset/sysreset_sandbox.c
@@ -50,7 +50,7 @@
 	 * (see the U_BOOT_DEVICE() declaration below) should not do anything.
 	 * If we are that device, return an error.
 	 */
-	if (state->fdt_fname && !dev_of_valid(dev))
+	if (state->fdt_fname && !dev_has_ofnode(dev))
 		return -ENODEV;
 
 	switch (type) {