sh-pfc: Don't take the sh_pfc spinlock in sh_pfc_map_gpios()

The sh_pfc_map_gpios() function is only called at initialization time
when no other task can access the sh_pfc fields. Don't protect the
operation with a spinlock.

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