dm: core: Add dev_get_uclass_priv() to access uclass private data

Add a convenience function to access the private data that a uclass stores
for each of its devices. Convert over most existing uses for consistency
and to provide an example for others.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/gpio/s5p_gpio.c b/drivers/gpio/s5p_gpio.c
index 0a245ba..49b1054 100644
--- a/drivers/gpio/s5p_gpio.c
+++ b/drivers/gpio/s5p_gpio.c
@@ -296,7 +296,7 @@
 
 static int gpio_exynos_probe(struct udevice *dev)
 {
-	struct gpio_dev_priv *uc_priv = dev->uclass_priv;
+	struct gpio_dev_priv *uc_priv = dev_get_uclass_priv(dev);
 	struct exynos_bank_info *priv = dev->priv;
 	struct exynos_gpio_platdata *plat = dev->platdata;