sh-pfc: Make struct pinmux_gpio enum_id field const

This ensures that the field is not modified, which is a prerequisite for
the rest of the PFC refactoring work.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/sh-pfc/sh_pfc.h

@@ -36,7 +36,7 @@ enum {
 #define PINMUX_FLAG_DREG            (0x3f << PINMUX_FLAG_DREG_SHIFT)
 
 struct pinmux_gpio {
-	pinmux_enum_t enum_id;
+	const pinmux_enum_t enum_id;
 	pinmux_flag_t flags;
 	const char *name;
 };