drivers: pinctrl: Support pinctrl with GD32VF103

Change the settings to support pinctrl on the GD32VF103.

- Split soc/arm/gigadevice/common/pinctrl_soc.h
  and put it into include/dt-bindings.
- Leave some definitions that can't handle with device tree compiler
  in pinctrl_soc.h.
- Remove dependency to SOC_FAMILY_GD32 because always enabled it
  if GD32_HAS_AF(IO)_PINMAX was selected.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>