pinctrl: sh-pfc: Improve core and user API separation

The Renesas Pin Function Controller uses two header files:
  - sh_pfc.h, for use by both core code and SoC-specific drivers,
  - core.h, for internal use by the core code only.

Several SoC-specific drivers include core.h, as they need the sh_pfc
structure, which is passed explicitly to the various SoC-specific
callbacks, and used there.

Hence move its definition from core.h to sh_pfc.h, and remove the
inclusion of core.h from all SoC-specific files.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Linus Walleij <linus.walleij@linaro.org>