gpio: syscon: allow fetching syscon from parent node

Syscon nodes can be a simple-mfd and the syscon-users then be declared
as children of this node. That way the parent-child structure can be
better represented for devices that are fully embedded in the syscon.

Therefore allow getting the syscon from the parent if neither
a special compatible nor a gpio,syscon-dev property is defined.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Levin Du <djw@t-chip.com.cn>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-syscon.c

@@ -200,6 +200,8 @@ static int syscon_gpio_probe(struct platform_device *pdev)
 	} else {
 		priv->syscon =
 			syscon_regmap_lookup_by_phandle(np, "gpio,syscon-dev");
+		if (IS_ERR(priv->syscon) && np->parent)
+			priv->syscon = syscon_node_to_regmap(np->parent);
 		if (IS_ERR(priv->syscon))
 			return PTR_ERR(priv->syscon);