pinctrl: sh-pfc: Move SoC-specific forward declarations to sh_pfc.h

With C=1:

    drivers/pinctrl/sh-pfc/pfc-emev2.c:1695:30: warning: symbol 'emev2_pinmux_info' was not declared. Should it be static?
    drivers/pinctrl/sh-pfc/pfc-r8a7779.c:3888:30: warning: symbol 'r8a7779_pinmux_info' was not declared. Should it be static?

Note that there are more warnings on SH.

The sh_pfc_soc_info structure is defined in sh_pfc.h, while all forward
declarations for the SoC-specific versions are in core.h.
Move the forward declarations from core.h to sh_pfc.h to fix this.

Reported-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Linus Walleij <linus.walleij@linaro.org>