arm64: rockchip: rk_gpio: Fix pin number

The maxpin counter starts at 0, fix one by one error.
This is still not totally correct for some banks in some SoC that have
fewer pins but this will be dealt with in another commit.

MFC after:	3 days

sys/arm64/rockchip/rk_gpio.c

@@ -194,7 +194,7 @@ rk_gpio_pin_max(device_t dev, int *maxpin)
 
 	/* Each bank have always 32 pins */
 	/* XXX not true*/
-	*maxpin = 32;
+	*maxpin = 31;
 	return (0);
 }