drm/aspeed: Look up syscon by phandle

This scales better to multiple families of SoC. The lookup by compatible
can be removed in a future change.

The fallback path is for the ast2500 platform only. Other platforms will
be added with the new style, so they won't need fallback paths.
Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209123734.130483-2-joel@jms.id.au

drivers/gpu/drm/aspeed/aspeed_gfx_drv.c

@@ -103,6 +103,7 @@ static int aspeed_gfx_load(struct drm_device *drm)
 {
 	struct platform_device *pdev = to_platform_device(drm->dev);
 	struct aspeed_gfx *priv = to_aspeed_gfx(drm);
+	struct device_node *np = pdev->dev.of_node;
 	struct resource *res;
 	int ret;
 
@@ -111,10 +112,13 @@ static int aspeed_gfx_load(struct drm_device *drm)
 	if (IS_ERR(priv->base))
 		return PTR_ERR(priv->base);
 
-	priv->scu = syscon_regmap_lookup_by_compatible("aspeed,ast2500-scu");
+	priv->scu = syscon_regmap_lookup_by_phandle(np, "syscon");
 	if (IS_ERR(priv->scu)) {
-		dev_err(&pdev->dev, "failed to find SCU regmap\n");
-		return PTR_ERR(priv->scu);
+		priv->scu = syscon_regmap_lookup_by_compatible("aspeed,ast2500-scu");
+		if (IS_ERR(priv->scu)) {
+			dev_err(&pdev->dev, "failed to find SCU regmap\n");
+			return PTR_ERR(priv->scu);
+		}
 	}
 
 	ret = of_reserved_mem_device_init(drm->dev);