kernel-doc: fix new warning in regulator core

Fix new kernel-doc warning:

Warning(drivers/regulator/core.c:2741): No description found for parameter 'of_node'
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Cc:	Liam Girdwood <lrg@ti.com>
Cc:	Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/regulator/core.c

@@ -2731,6 +2731,7 @@ static void rdev_init_debugfs(struct regulator_dev *rdev)
  * @dev: struct device for the regulator
  * @init_data: platform provided init data, passed through by driver
  * @driver_data: private regulator data
+ * @of_node: target open firmware device structure (may be NULL)
  *
  * Called by regulator drivers to register a regulator.
  * Returns 0 on success.