pinctrl: rockchip: constify rockchip_pin_ctrl for RK3399

There's no need to modify private data from the controller, so let's
make that struct const.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/drivers/pinctrl/rockchip/pinctrl-rk3399.c b/drivers/pinctrl/rockchip/pinctrl-rk3399.c
index b7a5092..928ed59 100644
--- a/drivers/pinctrl/rockchip/pinctrl-rk3399.c
+++ b/drivers/pinctrl/rockchip/pinctrl-rk3399.c
@@ -279,7 +279,7 @@
 			   ),
 };
 
-static struct rockchip_pin_ctrl rk3399_pin_ctrl = {
+static const struct rockchip_pin_ctrl rk3399_pin_ctrl = {
 	.pin_banks		= rk3399_pin_banks,
 	.nr_banks		= ARRAY_SIZE(rk3399_pin_banks),
 	.grf_mux_offset		= 0xe000,