dm: core: Fix devfdt_get_addr_ptr return value

According to the description of devfdt_get_addr_ptr, this function should
return NULL on failure, but currently it returns (void *)FDT_ADDR_T_NONE.

Fix this by making devfdt_get_addr_ptr return NULL on failure, as
described in the function comments. Also, update the drivers currently
checking (void *)FDT_ADDR_T_NONE to check for NULL.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/timer/ast_timer.c b/drivers/timer/ast_timer.c
index 3838601..9f28cbf 100644
--- a/drivers/timer/ast_timer.c
+++ b/drivers/timer/ast_timer.c
@@ -65,8 +65,8 @@
 	struct ast_timer_priv *priv = dev_get_priv(dev);
 
 	priv->regs = devfdt_get_addr_ptr(dev);
-	if (IS_ERR(priv->regs))
-		return PTR_ERR(priv->regs);
+	if (!priv->regs)
+		return -EINVAL;
 
 	priv->tmc = ast_get_timer_counter(priv->regs, AST_TICK_TIMER);