pinctrl: intel: Fix kernel doc format, i.e. add return sections

Kernel doc validator is not happy:

  pinctrl-intel.c:865: warning: No description found for return value of 'intel_gpio_to_pin'
  pinctrl-intel.c:904: warning: No description found for return value of 'intel_pin_to_gpio'
  2 warnings

Add return sections to the kernel documentation of the above mentioned
functions.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>