Documentation: debugfs: Document debugfs helper for unsigned long values

When debugfs_create_ulong() was added, it was not documented.

Fixes: c23fe831

 ("debugfs: Add debugfs_create_ulong()")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/20191021150645.32440-1-geert+renesas@glider.be
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>