gpio: sifive: Fix -Wint-to-pointer-cast warning

dev_read_addr() returns a value of type fdt_addr_t which is a 64-bit
address and plat->base is a pointer. In a 32-bit build, this causes the
following warning seen when building sifive-gpio.c:

  warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

Change to use dev_read_addr_ptr().

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/drivers/gpio/sifive-gpio.c b/drivers/gpio/sifive-gpio.c
index abd1f62..151f484 100644
--- a/drivers/gpio/sifive-gpio.c
+++ b/drivers/gpio/sifive-gpio.c
@@ -157,13 +157,11 @@
 static int sifive_gpio_of_to_plat(struct udevice *dev)
 {
 	struct sifive_gpio_plat *plat = dev_get_plat(dev);
-	fdt_addr_t addr;
 
-	addr = dev_read_addr(dev);
-	if (addr == FDT_ADDR_T_NONE)
+	plat->base = dev_read_addr_ptr(dev);
+	if (!plat->base)
 		return -EINVAL;
 
-	plat->base = (void *)addr;
 	return 0;
 }