kernel-doc: Allow anonymous enum

In C is a valid construction to have an anonymous enumerator.

Though we have now:

  drivers/pinctrl/intel/pinctrl-intel.c:240: error: Cannot parse enum!

Support it in the kernel-doc script.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>