i2c: mux: Allow muxes to work as children of i2c bus without i2c-parent

For mux check if the parent is already a device of UCLASS_I2C and if yes
just use that. Otherwise see if someone specified an i2c-parent phandle.
This mimics the behavior found in the Kernel, as it removes the
requirement to explicitly specify a i2c-parent phandle.

Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: u-boot@lists.denx.de

drivers/i2c/muxes/i2c-mux-uclass.c

@@ -86,6 +86,16 @@ static int i2c_mux_post_probe(struct udevice *mux)
 	debug("%s: %s\n", __func__, mux->name);
 	priv->selected = -1;
 
+	/* if parent is of i2c uclass already, we'll take that, otherwise
+	 * look if we find an i2c-parent phandle
+	 */
+	if (UCLASS_I2C == device_get_uclass_id(mux->parent)) {
+		priv->i2c_bus = dev_get_parent(mux);
+		debug("%s: bus=%p/%s\n", __func__, priv->i2c_bus,
+		      priv->i2c_bus->name);
+		return 0;
+	}
+
 	ret = uclass_get_device_by_phandle(UCLASS_I2C, mux, "i2c-parent",
 					   &priv->i2c_bus);
 	if (ret)