gpio: Allow to print pin's label even for pin with GPIOF_FUNC function

Currently, if pin's function is GPIOF_FUNC, only "func" if displayed
without any other information. It would be interesting, if information is
available, to indicate which pinmuxing's name is used.

For example, for STM32 SoC's based platform, "gpio status" command
output :

   before
    Bank GPIOZ:
      GPIOZ0: unused : 0 [ ]
      GPIOZ1: unused : 0 [ ]
      GPIOZ2: unused : 0 [ ]
      GPIOZ3: unused : 0 [ ]
      GPIOZ4: func
      GPIOZ5: func
      GPIOZ6: unused : 0 [ ]
      GPIOZ7: unused : 0 [ ]
      GPIOZ8: unknown
      GPIOZ9: unknown
      GPIOZ10: unknown
      GPIOZ11: unknown
      GPIOZ12: unknown
      GPIOZ13: unknown
      GPIOZ14: unknown
      GPIOZ15: unknown

   After
    Bank GPIOZ:
      GPIOZ0: unused : 0 [ ]
      GPIOZ1: unused : 0 [ ]
      GPIOZ2: unused : 0 [ ]
      GPIOZ3: unused : 0 [ ]
      GPIOZ4: func i2c4-0
      GPIOZ5: func i2c4-0
      GPIOZ6: unused : 0 [ ]
      GPIOZ7: unused : 0 [ ]
      GPIOZ8: unknown
      GPIOZ9: unknown
      GPIOZ10: unknown
      GPIOZ11: unknown
      GPIOZ12: unknown
      GPIOZ13: unknown
      GPIOZ14: unknown
      GPIOZ15: unknown

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>