pinctrl: pinctrl_stm32: Populate uc_priv->name[] with pinmux node's name

Populate uc_priv->name[] with pinmux node's name in order to indicate
the pinmuxing's name in case GPIO is configured in alternate.

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: Patrick Delaunay <patrick.delaunay@foss.st.com>