sh-pfc: Move GPIO registers access functions to gpio.c

Move the sh_pfc_setup_data_regs(), sh_pfc_setup_data_reg(),
sh_pfc_get_data_reg(), sh_pfc_read_bit() and sh_pfc_write_bit()
function to gpio.c as they belong to the GPIO implementation. Inline
sh_pfc_read_bit() and sh_pfc_write_bit() in their only call location.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>