lib: utils/gpio: Use fdt_driver for initialization

FDT gpio drivers have an extra .xlate operation, so they need to embed
the `struct fdt_driver` inside the subsystem-specific type. The gpio
subsystem always initializes the driver for a specific DT node.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

include/sbi_utils/gpio/gpio.h

@@ -40,7 +40,7 @@ struct gpio_pin {
 /** Representation of a GPIO chip */
 struct gpio_chip {
 	/** Pointer to GPIO driver owning this GPIO chip */
-	void *driver;
+	const void *driver;
 	/** Uniquie ID of the GPIO chip assigned by the driver */
 	unsigned int id;
 	/** Number of GPIOs supported by the GPIO chip */