of: add missing documentation for of_platform_populate()

15c3597d (dt/platform: allow device name to be overridden) added a
lookup parameter to of_platform_populate() but did not update the
documentation.  This patch adds the missing documentation entry.

Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Jiri Kosina <trivial@kernel.org>
Signed-off-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/of/platform.c

@@ -436,6 +436,7 @@ EXPORT_SYMBOL(of_platform_bus_probe);
  * of_platform_populate() - Populate platform_devices from device tree data
  * @root: parent of the first level to probe or NULL for the root of the tree
  * @matches: match table, NULL to use the default
+ * @lookup: auxdata table for matching id and platform_data with device nodes
  * @parent: parent to hook devices from, NULL for toplevel
  *
  * Similar to of_platform_bus_probe(), this function walks the device tree