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/at91_gpio.c b/drivers/gpio/at91_gpio.c
index 22fbd63..75a32ee 100644
--- a/drivers/gpio/at91_gpio.c
+++ b/drivers/gpio/at91_gpio.c
@@ -511,7 +511,7 @@
 {
 	struct at91_port_priv *port = dev_get_priv(dev);
 	struct at91_port_platdata *plat = dev_get_platdata(dev);
-	struct gpio_dev_priv *uc_priv = dev->uclass_priv;
+	struct gpio_dev_priv *uc_priv = dev_get_uclass_priv(dev);
 
 	uc_priv->bank_name = plat->bank_name;
 	uc_priv->gpio_count = GPIO_PER_BANK;