pinctrl: sh-pfc: Introduce PINCTRL_SH_FUNC_GPIO helper symbol

Pinctrl drivers for SuperH platforms use legacy function GPIOs.
Currently this support is compiled in based on the SUPERH platform
dependency, which hinders the introduction of compile-testing support
for the affected pinctrl drivers.

Introduce a new Kconfig symbol PINCTRL_SH_FUNC_GPIO, which is
auto-selected when needed.  This symbol in turn selects
PINCTRL_SH_PFC_GPIO, to reduce the number of per-driver selects.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>