sh-pfc: Merge PFC core and gpio

The PFC core calls the gpio module gpiochip registration in its
register_sh_pfc() function, itself called at arch initialization time.
If the gpio module isn't present then the gpiochip will never be
registered.

As the gpio module can only be present at arch initialization time if
it's builtin, there's no point in allowing to build it as a module. Make
it a boolean option, and initialize it synchronously with the core if
selected.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>