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>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>